I just built my vacuum forming setup and first tried it out with those acrylic sheets you can get at home depot before ordering the HIS sheets, and it does work. It definitely takes a little longer to heat up, and you have to be quick, but I was able to form it nicely around my mold.
Also, it has a tendency to crack when cooling if it was stretched too much, so forming smaller cases is better. At least in my experience.
Good luck, whatever you decide.