The etching / stamping backups seem to be the most durable and cost effective as per Lopp's tests:

Conversely, backups with moving parts (like the casette models) are less durable and way more expensive... Easier to set up I suppose.

You could go cheaper still by buying stainless steel business cards and an engraving pen.

Here you have a very good manual to DIY, is in spanish but easy to transalate. Made by @lunaticoin and his team.

This one is quite nice. https://diyseed.net/ You end up with a plate with a bunch of marks. Not showing alphabet and positions gets you an extra layer of protection.

Mason jar lids work fine. I’ve never actually heard anyone recommend this, but I’ve been using them for awhile.

I even flamed mine with a torch, and it was still metal at the end.

I am such the genius.

None yet. Though considering fire proof envelopes (yep, they are a thing)

I use stainless steel washers and a metal stamp kit.

I can tell you that a friend of mine bought metal plates from a manufacturer I can't recall now. They were the type that uses a metal punch to mark individual letters. He discovered that the ink used to mark the squares and letters began wearing off before he had even finished punching out his seed words.

Caveat emptor

none, seems like just a novelty item to me. totally unnecessary for the vast majority of people.

none, I just buy more sats instead of metal plates.