Hmmm well I use the rolled filter pad the one from Optimum Aquarium. It is the kind that has a blue side and a white side. It works well. And it only cost me like 3 dollers for a HUGE piece. P.S. The Perks are doing great!!
When I was using a pre-filter, I literally used a sock; I got package of white socks for wally world for about $3.50. They worked suprisingly well, but I stopped using them because I found that they trapped a good bit of 'pods as well as detritius.
Just be sure to change whatever prefilter you're using every week or so; you don't want the detrius trapped in the prefilter to break down and repollute your water .