You should provide a picture of the tank and area.
1. How you want to mount it (from the ceiling, rest on the tank, suspended from a wall.
2. Are you looking for a canopy style (enclosed) or just a frame with sides.
3. Its important to understand how you access the top of the tank, how deep the tank is, and how easy it is to move the canopy if needed.
4. What material
I prefer to hang it from the ceiling, my build thread has some of what was done.
I used some L aluminum from home depot for "tracks" that I mounted to 2x 2x4s across the width of the tank and the aluminum along the length (radions). The 2x4s had hooks that were connected by chains to hooks in the ceiling. The chain allowed me to customize the height of the fixture by moving which chain link was on the ceiling hook. This also made it easy to raise the whole fixture for tank maintenance.
I used 1/4' PVC sheet with wood texture (4x8 sheet for $60 at home depot) and screwed those to the sides of the fixture (2x4s) to make it pretty.
The light was independent of the canopy, which rested on the edge of the tank frame. I made it with 2x4 that rested on the frame, then screwed 1x6 or 1x8" to that to cover the black plastic frame. this was very light so if I needed to access the tank I could just slide it off without dealing with the lights etc.