If you are trying to make a canister, you wont be able to seal the top properly. If you make it into a sump, remember, the pump will be in the bottom, then the bio-media and finally, the mechanical. This is working from the bottom up, of course. The main problem I have had is plumbing the pump. Exterally plumbed return pump solves this, but be sure to use the proper bulkheads with adequate seals. The material generally used for trash cans and storage containers in general wont except most glues or adhesives. Also, the more water being held in the container will cause the container to bulge outward. Again, if you are going to plumb an external pump, use the proper bulkhead/seal. The only other problem I have had is not knowing "how its doing" in terms of when to clean it or if the media is clogged because you cant see into it. You have to take it apart to know.
Anyway, I have played around with this idea amongst others and it is fun to try and make things work on the cheap. Good luck.