If you're using fiberglass, the best idea would be to make it thick, but I would advise caution in this project. The reason airsofters can get away with homemade armor is that their projectiles are smaller. Paintballs travel slightly slower than airsoft rounds, but have around ten times the mass. They can punch through a lot of things an airsoft round might bounce off because the energy transfer is greater. Keep that in mind, and test your armor before you take it onto the field. But no matter what you do, DO NOT use a custom helmet on the field. Even if you take your lens from a paintball mask or other certified helmet, it does not mean that your helmet is certified for play.