I got tired of paying 3 bucks a filter for my whisper 5-15 fish filter for my sons goldfish and the filters are becoming harder to find in the area I live in so I decided to make my own for what I later figured to be less then 15 cents a filter. Im sure this same methode could be used on other like filters/pumps that use this same style filter as well, you will just have to adjust size and fit.
Step 1: Make a frame
The replacement filters I have been buying didn't have any internal support. Some of them do and some of them don't. The last one I had did not so I had to come up with some sort of internal frame for the new filter. I found that a light switch cover to fit perfectly.
Step 2: Cutting Frame
Now some light switch covers are made from a hard plastic and you wont be able to cut them with a razor knife without cracking. The one I had in the garage was made of some sort of soft vinyl and I found I could cut it very easy with a razor knife. I cut it so there was about 1/8-1/4 inch of plastic left all the way around the edge. You might be able to buy a one of those square light switch covers that already have a large square hole cut them in or maybe even use a plug cover and break out or cut out the center piece. Whatever you use make sure it will allow for the most water flow for best filtering.