I have done this before with a chunk of 3/16 HDPE (High Density Polyethylene). I built a mold/form out of laminated particle board and carefully heated the plastic with a heat gun and with a welding glove on one hand pressed the HDPE down onto my form. It took a while and I had to use lots of clamps to hold the edges down, but looked pretty good when it was done. Or how about just a flat sheet of aluminum? Your design might have to be basic to avoid lots of welding, but the aluminum is pretty readily available.
