short answer is don't do it unless you're okay with making a mess and destroying the body.
Proper way is to build a block mold, heat the plastic in an oven then vacuum form the plastic over the mold, this guy explains the steps here:
Only problem is you need a clear flat plastic sheet, you...
You can't modify a lexan body without compromising the integrity of the shell. If it will be a "shelf queen" then fine, but expect the shell to crack and will be destroyed the first time you roll over :(
There are flat bed options out there for various classes:
https://rccarkings.net/bodies.htm