They look like they came from a pet store.
If they are glass you can patch the hole by siliconing some glass over it.
Using it for a canister is a better idea. Large tanks like that need larger filters than most HOB style filters can really manage. And I am not sure how well a HOB would do on the back of the tank, once you remove some of the trim. Might be better not to stress that part.
I could see a riparium set up with the different heights of the sides, back and front. Maybe as full of water as they will hold, but use tall driftwood and emersed plants, a taller background that sticks up above the tank... things that will make use of the taller sides.