Yep....I've got the design almost perfected (well, almost perfected on paper anyways). I'm going to use an 18 gallon rubbermaid for the sump, and one of those 3 drawer storage thingys (the white ones by rubbermaid) to hold the media.
The top drawer wil have my foam, the second drawer will have the carbon, and the third (very big) drawer will have the bioballs. I'm contemplating using a series of biowheels instead of balls...not sure what I'm going to do yet depending on performance.
THat whole thing will sit inside the sump tank. The sump tank will contain my CO2 injectors, heater, and pump, which will shoot the water back to the tank.
I decided on drawers because I can easily access the carbon and the foam for clean/change purposes. Also, it kind of follows the same concept as some canisters I've seen, using gravity as a guide for waterflow. The only problem I think I'll have is noise reduction, but I've got some basic ideas for that as well.
I really like this idea for a bunch of reasons. First, I can get that big ugly heater out of my tank, and hide it but still heat the water. Also, I'll be able to take off the power filters, freeing up space for my homemade canopy, and third, I can use the warm sump water to insulate my homemade co2 bottles and make the yeast work faster.
The only disadvantage that I see right now is that I'll have the big ugly overspill contraption showing in the tank. I'm working on a design of all clear plastics and some camoflage to hid it a bit though. The second disadvantage is that it'll cost some money, but I SHOULD be able to do the filter and canopy for far less than a good canister will cost.
Once I start, I'll throw some pics up in the DIY section.
. noisy, hard to prime, ugly black piping, expensive replacement bags, etc. 
