I have always seen them curl real bad, so after making a couple and the same thing happening, now when I make them I put a very, very thin sheet of metal in the back and then cover the back with deer skin and lace the front and back together. You could use lexan/acrylic. I just always have some brass or metal laying around. And if you use a metal you can bend it to fit the tank!
I have done them before with just a leather backing, but after getting baked in the sun, they did curl up...so now I always use a stiff backing. Kydex/acrylic/metal, all should be good. Then use the softest leather you can get locally, sheep,deer,rabbit,maybe even a piece of 2-3oz with the kip side out would probably be ok.
Maybe just go to autozone and get a real leather shammy...they are normally deer. And under 5-7 bucks. bu-dum-ch...hahahaha
Thank you , thank you I'm here all night...lol!
Also another idea that is super cheap is a heavy Felt like 1/8" ; 2-3 bucks tops.
Maybe someone else has another idea, but I think that pretty much nails it on the head!
For sure use a stiff backer middle plate!