For screwing directly into plastic I'm a fan of "plastite" screws. ( http://www.mcmaster.com/#plastite-screws/=sdaz58 )

The other option that works pretty well is "nutstrip" ( http://www.fingertechrobotics.com/pr...?prod=NutStrip ) as you just have to match the hole pattern on whatever you're bolting together and you can do all of your panels quickly as 2d profiles which means you can quickly make spares and change them out when they get damaged.