When I feel the need to protect fry, I get two wide pored sponges. Always two. I cut holes in the top of them, so that the intake pipe can fit without being jammed. Usually, that puts the hole off centre. I use large sponges - that's important. Aquaclear replacement sponges are ideal.
The holes only cover the depth the filter strainer fits in. They're pockets and don't go all the way through. I jam them on the inlet and fire it all up. If I've cut them badly, they interfere with the placement of an inlet for a HOB, and sometimes you have to play with it.
Whatever size filter I use I use the correspondingly sized Aquaclear sponge, even for other brands. I don't try to hide it. This is for fry tanks, which aren't well scaped anyway. Fish will start to feed off these sponges, as they rapidly attract a bacterial fauna. Dwarf Cichlids will even lead their fry to them.
Every week or two, I switch the sponges out, rinse the dirty one and often sit it in an out of view spot in another tank. You might as well profit from the bacteria and archaea on it, right? Dry it out and it's just mechanical.
My system now is largely air driven, but when I raised a lot of dwarf Cichlids, I used a lot of power filtration, and that's how I'd deal. I wouldn't use prefilters on community tank filters because I have never had a problem with fish going up them. And I only keep small fish. If I had loaches, I might prefilter everything.
