any glass 5mm+ thick should be fine, this will block most of the bad UV. the second piece isnt really necessary. check out the design of some halide pendants of the same wattage, these should be a good gauge of what is "safe" - thickness, distances etc. it would pay not to let any of the unfiltered light out of the reflector - whether it be into the tank, room or your eyes. theres a recent thread on this site where a guy had to have some medical attention due to "arc eye" resulting from an unshielded DE halide.