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Hi,
The hardware side is straightforward, but the software side of the system you want is a tremendous amount of work and potentially bug prone. I'd recommend using an off the shelf solution if possible, given the edge cases for such an implementation result in either unauthorized access or legitimate access blocked.

To start, what you want is probably 13.56MHz RFID. It's close range and used for most card reader systems and keyfobs. The reading distance is generally determined by the power in the reader antenna. There is no power source in the RFID chip itself, so it activates as soon as any part of the the chip's mini antenna gets in range of the reader.

That said, you are intent on the DIY route, here is how I might build such a system:

1x Local network authentication server.

• Off the shelf machine, preferably running Linux. Doesn't need to be powerful.
  
• User Interface, possibly web based, so that network clients (e.g. the front desk) can manage credentials.
  
• DB that stores credentials. Could be RDBMS (Mysql, Postgres), or flat file. Depends on requirements. You want to store a unique identifier on the card here, then associate that ID with user credentials and which doors they can access.
  
• Network service that responds to access requests. When a reader picks up a new card, it sends the card ID and some assigned ID for the door. It gets some response, probably a binary yes or no for whether the user can access this specific door.
  
  There are many moving parts on the software side. This could be months of work for a single person depending on the requirements and how much time they spent on it.
  
  

  RFID Reader (per door)
  

• 1x Mifare RC522 reader. Reads cards at a distance of up to about 2cm-4cm.
  
• 1x Arduino with wifi shield. Try not to get a cheap wifi shield because the cheaper they are the more time they take to work with.
  
• 1x plastic enclosure that will house the whole thing. (Something like this might work, although not sure with the RC522 on top). Enclosure cannot be metal.
  
• (Optional) 1x piezo speaker (or some other speaker type). You want audible feedback for the card making contact with the door, especially because of the short range of the reader (~2cm).
  
• (Optional) 1x RGB LED that you can set to green or red based on whether the badge in was accepted or rejected. You could probably get by without this, if you at least have some feedback with the speaker.
  
• 1x USB reader to connect to clients. Haven't found the one I bought, but this one looks okay. This device will let you write credentials to a card. Visitor comes in, person at front desk places card on reader, sets the card data you want. It would also register the card's unique ID and you would use that to grant system access.
  This is what a Mifare looks like attached to an Arduino. The breadboard is only involved for testing so I didn't have to solder the mifare. Was still testing different configurations.
  http://imgur.com/a/1eaQ3
  
  

  Advice
  

  Part wise the above is cheap, but implementation is really time consuming. It also does not address actually opening the doors. I'm assuming you have electronic locks. Factoring in time to build and test I would probably go with something like this (quick google search, sponsored result): http://www.maglocks.com/rf-m8500-mortise-rfid-card-hotel-lock-with-built-in-deadbolt-by-digi-electronics-for-209.html. Surely not the best on the market but if I found one in 30 seconds, I'm sure there's one out there specifically for your purposes.
  

Homelink should be pretty straightforward, assuming that the code is fixed and not rolling (in which case you would probably need access to the "learn"/"programming" button on the main controller).

​

RFID cloning is a little trickier. If the complex is using 125khz prox, this is probably your best option:

https://tinylabs.io/keysy/

https://www.amazon.com/Keysy-RFID-Duplicator/dp/B07D7K2LCB

​

It's a relatively new product aimed at pretty much your exact use scenario. I've had good luck with it on 125khz, but if the complex is using a more advanced (13.56mhz) system, you will need something like the Proxmark3 and a good bit of time to figure out how to use it.

1. you sure those are the right format for cloning your campus’ proximity card? There are multiple standards using 125khz, and the amazon page you linked is for EM4100 with a read-only ID
   
   
2. I did find EM4100 stickers that claim a 0.3mm thickness: https://www.amazon.com/dp/B07PCJKPZZ/ref=cm_sw_r_cp_api_i_z5.QDbB9Z2XKW
   
   
3. If your campus uses HID Prox (incredibly common), you’ll want to look for T5577 which can emulate HID Prox, effectively cloning them.
   
   
4. Ultimately your question is about form factor, and aside from the required structure or thickness of the rfid chip/antenna, what is available is hard to qualify a priori. It’s really a matter of doing a lot of searching across lots of vendors. I would suggest using the words “thin” and “sticker” to get results nearer what you want. Obviously there are lots of pvc card and keychain fob styles available.

I got the ring 3D model here
I got the tags from here
I dropped the tag in acetone, and let it sit for a couple hours. After I pulled it out, I re-wrapped the coil tighter and superglued it to the ring. Then I put some black electrical tape around the chip for added protection.
It actually works pretty well, but the range was shortened quite a bit.

HFeng (Pack of 100) 125KHz RFID... https://www.amazon.ca/dp/B07DYCJDXQ?ref=ppx_pop_mob_ap_share

Here's the link to the on Amazon

What is important is the Card. The Standard you Need is NFC Forum Tag Type 5. Based on ISO 15693 Standard. You will probably have to make your own antenna for a reader that supports read of ISO 15693 cards.

Two Chips support the Standard. The NXP iCode SLIX and the STMicroelectronics ST25TV series. The only thing is that there are several out there but a lot is based on the Antenna in the tag. Several do not mention distance obtainable.

rfidcard.com offer type 5 cards. https://www.rfidcard.com/product-category/nfc-card/nfc-tag-type-5/ But they sell bulk. Plus all their NXP SLIX cards all list up to 1.5 Meters but their ST25TV cards both say read at 10cm.

​

Now Yarongtech has Icode Slick 2 Cards but they do not list distance. This card has the right chip and ISO it will just take some experimenting to see if it realizes it's full distance.

https://www.amazon.com/ISO15693-13-56mhz-Blank-iCode-White/dp/B07DB2VDR8

The hardest part is finding a reader that will read ISO 15693 Type 5 cards and has enough power to do it at the full distance.

This is probably too much work for a single small client... just throwing it out there. Something like this works with lots of (but not all) smartphones:

http://www.amazon.com/RFID-ME-Android-Powered-Devices/dp/B007KXC1NO

There's an Android app with source code available that shows how the basic reading/writing works:

https://github.com/mti-rfid/RFID_ME_GUI_APP

(I have this setup, and it works fine close to the tags.)

It doesn't sound like you need much range, so barcodes and NFC tags might be good options too...

Something like https://www.amazon.com/Ceramic-Eclipse-NFC-Ring-Programmable/dp/B01B2319OY/ assuming the door readers work with NFC.