Reddit reviews Remo HD-8510-00 Fiberskyn Frame Drum, 10"

I honestly think its gonna be cheaper for you to get a $100 Chinese banjo off Ebay or craigslist. Its gonna sound better too. If you take the neck and tuners from a guitar the string spacing is gonna be off and its gonna be difficult to switch to an actual banjo after. If you plan to use a tambourine as the pot you still need a tailpiece, bridge, armrest. I don't think its feasible to build something that resembles a banjo for leas than the price of a very cheap Chinese banjo. The material costs are gonna be close and not to mention all the timel its gonna take and the end product won't be something worth the effort.

That being said. If you still wanna build one, look for a 10" frame drum or bodhran.
https://www.amazon.com/Remo-HD-8510-00-Fiberskyn-Frame-Drum/dp/B0002F7KGK/ref=mp_s_a_1_1?ie=UTF8&qid=1536944358&sr=1-1&pi=AC_SX236_SY340_FMwebp_QL65&dpPl=1&dpID=41LVEiM8ZAL&ref=plSrch

Ebay has cheap strings, tailpieces, and bridges. You're gonna need a nut that'll hold the strings at the proper height but that'll also ensure proper spacing. I think the hardest part is gonna be getting the neck done. You can either look for a cheap neck or try and modify the guitar neck to work. Modifying would require cutting down the width and length I believe.

Another concern would be the tension in the drum frame, the strings are going to be compressing the drum frame around the neck and tailpiece so you'll need to add a stiffening mechanism.

This is what I used for the banjo head. You may want to go with a 12" if going with a full scale length. My banjo is only 23" scale length.
http://www.amazon.com/Remo-Fiberskyn-Frame-Drum-10/dp/B0002F7KGK

Here are the plans I followed. The main thing I did different was how I attached the neck.
http://www.bluestemstrings.com/pageWineboxBanjo1.html