Clear material for UV Box
Polycarbonate is almost perfectly opaque to UV, so it's no wonder your setup isn't working. You can get special UV Transparent  (UVT) plexiglass, which will gain you energy in the 300 - 400 nm (long-wave). http://www.plasticgenius.com/2011/05/infrared-and-ultraviolet-transmission.html

You really need to find a data sheet for your resist, and see what wavelengths are required.

I would recommend that you don't use a plate for support of your board. Rather, make an adjustable framework which will just contact the board corners, and support the board that way.

And I agree that I'm not being helpful, but I can't resist (get it?) pointing out that sapphire will work excellently for your application. http://www.valleydesign.com/sapppic.htm