They do sell flat sheets of PVC but it's not widely available and if I remember right it's kind of expensive for the size.
Another option for flat sheet PVC is to buy, or scroung from a construction site, a lenght of tube, cut off a piece that will fit in your home oven, cut it down one side from end to end, heat your oven to 400 degrees and place the tube in the oven for about 5 minutes. PVC turns to rubber when it's heated and produces little if any smell, as long as you don't burn it. Once it's hot and flexable take it out and quickly, as it cools fast, place it on something smooth, flat and capeable of withstanding the heat, place a piece of plywood on top and set something heavy on top of the plywood, I used a few bags of mulch I had laying around.
When everything cools the PVC will be a flat sheet and you can cut it to any size and shap you want. You could use this same technique to form a piece to the shape of your arm if you wanted to make a wrist slate. Of course you wouldn't want to use your own arm as a mold, maybe your dive buddy's. :wacko:
Scott