You probably would want to use at least 3/4 inch, but it's rather expensive. ~$450 for a 4'x8' sheet, double for 1 1/4" material.

Yes, with 1/2", seems like you could get some flex. FWIW, when I needed some 1/2" for an application on my boat, I went to the local Laird Plastics and they sold me a quarter sheet (2'x4'). Still very expensive, but much less so. Not sure where you are located, but smaller plastics resellers may not sell partial sheets if they have to special order the material. Laird is a national chain and they didn't seem to have a problem selling quarter and half sheets. If you have a small plastics shop locally that does custom fabrication, go see if they have any remnants that will do the job. If so, you can probably get it at less than half price.