What filter are you using ? I have Aquaclear 70s, which have a fair size uptake tube. Finding prefilters for them isn't easy, but some of the sponge filters fit their uptake tubes and they work fine as prefilters.
Sponge filters come in several sizes, so if you can find one the right size, just slide the tube into the upper opening of the sponge. You'd have to plug the bottom of the hole with a bit of sponge or floss or something, as the hole goes all the way through on a sponge filter. Or if the uptake tube is long enough, simply leave the base on the sponge filter and leave the upper fitting for the air hose off. Then slip the uptake tube into the top hole.
Smaller uptake tubes may fit the sponge prefilters made for Marineland filters, which are sold in pairs usually. The three smallest Aquaclears fit these pretty well. Or you can make one yourself out of an Aquaclear or other brand of filter sponge, which I have also done.
Cut the sponge to the right length for what you want, then either cut an X shape in the sponge to slip the tube into, not quite all the way through, or cut out some sponge with scissors or a knife.
I know a guy who soaks a sponge in water, then freezes it in a baggie. Then he drills out the hole with a power drill. It's not a perfect hole, but much neater than using scissors or a knife.
No matter how you make the hole, so long as it slips on the tube and stays in place, it will work. Just be sure that if you end up with a a hole that goes all the way through, plug the end with something. A piece of the trimmed off sponge works best, or a piece of filter floss works too.
