Your much better off buying a Fluval 405...Rena xp3...or Eheim 2017. By the time you have a SAFE,EFFECTIVE, and RELIABLE powered cannister built you'll have more money in the project than any of the above filters cost...even at retail.
I know, thought about that. But I'm not trying to be coast effective, is just more like a fun project type thing. I like making things, so I just wanted to know certain specifics before I tried it.
I'd go with a 2g-3g tall, bulkhead connectors (or DIY ones) and make the trays out of eggcrate with pvc legs, make each level fit snugly so water can't channel around the filter material. Floss pre-filter, and scrubbies for the rest.