I thought so too. They're inexpensive so why are people going to the trouble of making one. Myself, I need to custom make one because I need quite a small, low one. I've already altered an existing sponge filter by replacing the original sponge with a square sponge I had on hand after cutting a hole in it to fit the rigid tube. Sponge filters are ugly things to begin with so by making it smaller, it's less of an eyesore.
