Great advice, thanks!
So step one should be making sure the back of the canopy is closed. Right now, about an inch at the back of the plexiglass is open to make room for the HOB. I'll work on closing that first. I should do that anyway, to keep my fish in the tank and off the floor!
Next step will be seeing what the heater does, and if it can't maintain 78 degrees, I'll replace it. Would 2x50w heaters work better? I've heard that can be the better route, but I'm not sure with the taller tank.
After that I will look into purchasing some foam board, that does sound more manageable than plywood!