I've built a whole bunch of them out of rubbermaids.
Basically the appropriate size rubbermaid bin with 3 holes in the bottom of it:
- one hole for input of water from the tank (via a pump in the tank)
- two holes for return of water via gravity to the tank.
Inside the rubbermaid is whatever media you want. For most I stack trays from the dollar store (top has fluff and bottom ones have bio balls, army men or other bio media). Water from the pump in the tank drops water onto the top tray.
Most of my tanks are 2'x2'x1', so I use 22"x14" x 6" rubbermaids on the front of the tank. I use 1" x 2" X 26" pieces of wood as rails to hold the filter and make it easy to slide open and closed...
Matt