So, I tried looking around the house for styrene (to fix the Generations Blitzwing shoulder problem), and I couldn't find any, alas. Does anyone know of some basic household alternatives that can prove to be a good substitute?
For your specific example, I just did the other trick for him and shaved the inside of the chest. But to answer your problem, I try to keep a bubble or two from figures to use as bits of plastic. Probably not the greatest thing to use but to add some details it works. For reference here's some Wheeljack molds with bubble plastic on the windshields. Painted they should look fine.
For Sale signs from Walmart. They're very inexpensive and essentially styrene, just with printing that sands off pretty easily. I've been using the stuff for years because, basically, hobby styrene isn't cheap. This stuff layers really well, it's thin and easily bendable, takes to all sorts of glues... Basically just good for everything.
Old credit cards also work well but need a bit of prep time to get sand numbers and the glossy layer off.
