I just finished a canopy for my 110g, which has the same footprint (18x48). It's listed in a thread on this section of the message boards.
In response to your questions, well, they both depend on a couple factors. Do you have a HOB filter, or is it reef ready? What sort of lighting do you have? And do you want your canopy to be fully enclosed, or will you leave the back open (or screened) for air flow?
I installed 2 12v fans in the top of my canopy. They do well to suck the hot air from the lighting system out through the top. My back section is open and vented with screening, so cool air has a path along the entire inside of the canopy. So far I haven't noticed any temperature increase, and my evaporation is still about the same: 1 g/day.