Need to be ASTM certified, no exceptions, a sheet of polycarb will probably not be ASTM certified for outdoor sports (ie. paintball). Same goes for fully sealed ballistics, even though it can stop a .22 which arguably carries more energy and travels faster than any paintball ever could if it's not ASTM certified it's not allowed.
I recommend JT Spectra or Flex 8's if you want something modular and remove panels (or even cut away at certain sections to get a better cheek weld).
http://www.paintballgear.ca/