Get a cheap tank off craigslist...55s are very common, you might even be able to get one for free. Get some 1/8" glass cut, silicone it in to divide off a section for the media. Put a drip tray above that. Then you just need a pump.
What is the viewing angle of the tank going to be?
I conceptualized a filter that I've never had the right conditions to use that would block out one side of the tank and use about 4" of the tanks length... leaving the front, back and other side available as viewing angles...
In short... build a false wall on one end of the tank about 4" from the actual aquarium's wall... stop the flase wall about 2~3" from the bottom of the tank... block the opening on bottom with eggcreate (placed behind the false wall)...
Waste will enter this built in filter from the bottom... Chambers could be built/designed to house media ultimately ending with a pump on top that pushes water back into the display tank via spray bars...