Not sure what you mean by glass canopy. A glass lid fit to the inside dimensions usually comes hinged, with a plastic strip that slides onto the glass for the final inch or two in the back. This plastic is cut away for filters and other tank hardware.
A canopy however, is placed on top of tank and lid, with the canopy resting on the outside dimensions of the tank. Thus, you can use the lid without removing the canopy. The canopy usually had a removable lid in the top so you can get to the tank lid directly on the tank. I have only seen canopies made out of wood, but I suppose they could be made from any manterial.
in the pic below you can see the matching wood canopy sitting on top of my tank. glass lids are below it to prevent evaporation and escapes.