A canopy is just a box that sits on top of the tank, nothing too special in the design. Just make it big enough to house the lights, with an air gap for heat dissipation (or maybe a fan or 2).
This is one example:
A Simple Build-It-Yourself Aquarium Canopy
Mine is similar, except I have it hinged only at the front 1/3 (with an acrylic lid) - easier to open, and you still have light to see with .... My entire back consists of 2 rails, & is totally open in between for air & heat movement.
I built the canopy so that the inside dimension of the canopy is 1/8" larger than the OUTSIDE dimension of the tank. The canopy then sits over the tank on cleats. This arrangement covers the black plastic trim of the tank (& match my DIY stand), plus the overlap prevents the canopy from slipping off the top. Others have the canopy exactly the same size as the tank so it sits on top. I don't think it is as stable, but might match some stands better.