Reddit reviews Yellow Brass 20 Gauge Sheet - 6" x 6"

There's a mess of similar solutions. Just search Amazon for steel wallet crypto. For some insane reason they're 70-150$ Cheapest is a titanium wallet at 20 but you need to punch your own letters, set of punches costs another 20. With that amount of exertion blockplate is probably your best price/performance option if you want a commercial product.

If you're not averse to DIY and some effort just buy or borrow a Dremel Engraver (~$15-20) or buy the letter punches and shop around for a brass plate, ~$8 here: https://www.amazon.com/dp/B00JDU1LCO/ref=cm_sw_r_cp_taa_dOZ9BbFGJ1H2C. Can be found smaller and cheaper. This way you have the tools to engrave/impress and can buy a few seed plates on the cheap if you would prefer not to keep all your eggs in one basket.

A more clever solution is to download this image of BIP-39 words, print it out, upload it to every cloud service and put it on every device. Then just engrave/impress the number attributed to the word on your backup. https://imgur.com/GT1StTL.jpg