Below is a picture of the finished canopy I built for my 6'x4' fibreglass tank. I built it to last. The framework is 2"x2". There's a removable central polycarbonate sheet and 6 plywood panels, all removable too. The underside has got a triple coat of exterior white paint and the top, as you can see is black.
Since this photo was taken I have put a nicrew led light fitting on top of the polycarbonate sheet and just to give it a finishing touch I build a housing for the light out of ply (with holes in the top to allow heat from the lights to escape). Then I painted the housing black to match the canopy and the actual tank.
You don't even need to be well versed in joinery to do something like this, it was very easy.
This is 4x8 for my almost done 500 plywood tank. Doors are made but not installed yet. It has 3 x 20 watt led food lights and 1 4 foot t5 uvb for turtles. Sides are 3/4 ply and top is 1/4. There are polycarbonate greenhouse panels covering the tank under the canopy.