With that type, you could get AC filter floss, the loose blue rectangles. And cut it a little larger than the top of that grate, cut a hole in the center and slip it on.
Make a couple of these and swap them out regularly, if you have no fish. Rinse, dry and reuse.
If you have fish, rinse the dirty pad and reuse it.
They don't make leaf baskets for those types of filters.