well, canopy usually refer's to a wooden box that holds lights and rests on the lip of the tank.
the glass top you speak of, I'd call a lid.
Are you having problems with the water getting too warm during the day?
I run my tank open top (no glass) with a canopy over that. even then I still have to run a fan inside to keep things at 80degrees.
Some people buy egg crate for fluorescent lighting from Home Depot type stores...basically a plastic grid of 1/4" squares, that's about 1/4" thick.
Former advisor and planted tank geek...life's moved on though.