Watch as i construct a hamburg mat filter out of a 2" piece of foam, a PVC tube, an airline hose, and an air stone. It's important to note that I used a check-valve on my airline which reduces the chance to having a back-siphon drain your tank - which is recommended whenever your air pump is below your water line.

If you can find open cell foam I would recommend you use it, however, the cheap foam remnants you find at discount stores works well also. These filters are so cheap no one really makes them, so have at it and save yourself some money!