[The plastic strainer that comes with my pumps. And if an anemone is roaming, I'll put a sponge prefilter over that, to assure that it can't be sucked into the pump's intake. The sponge has to be rinsed frequently until the anemone is finally affixed.]
[I wouldn't use foam...or anything that isn't rigid or it will get sucked into the pump. How many do you need? Maybe someone on the board has some extras running around that they aren't using.]
[I used foam sleeves for my maxijets for a long time with no problems. The only issue, as was pointed out, is that they need to be cleaned regularly, or the get gunked up and don't work as well.]