The canopy should be 10 - 12" tall. Dave's Custom makes his halide canopies 10" ( allows enough room for lots of equipment and to keep the halides 6-8" off the top of the tank) IF you make it taller it is easy to add blocks to drop the lighting back down, but if you start short... well there you are.
You only need glass sheilding in you use HQI bulbs w/o UV inhibitors. All most all mogul lamps have UV sheild built into them. As far as splash and spray concerns go, I run my mogul 250w 10" off the water and haven't had any problem in 4 yrs.