I would think that at 12 inches and braced you'd be ok with quarter inch in theory. 35 years ago I made a 72 inch long 17 inch tall out of quarter inch that held up for a few years and 3-4 moves before finally splitting seams. I wish I had braced it better and bumped up to 3/8ths.
I'm strongly considering doing a 16 footer this time. Either an 8 foot panel sandwiched between two 4 foot panels, or going for it and trying to butt join acrylic with weldon 40. In either case it'd have to be braced like mad, in my case I'd probably use half inch material either way because I'm looking more along the lines of 18-20 inches of water height. The length will make the bracing the imperative issue I believe... going longer doesn't technically increase pressure but it does want to increase the flexing of the material from what I gather. If you can make a rock solid frame you are ahead of the game.
