you can buy both ABS and Polypro plastic in 1/4" 4x8' sheets. The ABS is bumpy on one side and smooth on the other.You might have to search for the nearest distributor in your area. you might also want to invest in a good 'air welder' if you are going to fabricate much
The ABS is cheaper, but I find it is a little more brittle and heat welds can crack if you're not careful. Polypro is usually white, easy to heat weld, and welds seem tougher. I've used it a lot for mounting components, making little brackets, and battery boxes.... you can visit my site, and check the gallery sections....