I've never used an Aqua Clear 300 but I am familiar with them (I just peeked in the box at one the other day) You can use floss instead of a sponge or just stuff it around the sponge, wherever it fits the best, over or under,ya can't go wrong.
Just make sure you keep it away from the propellar, sometimes it can catch a piece and get knotted up and slow your filter down. Unless you use a bag to stuff your floss in. If it slows down your flow too much, just rearrange it until it works the best.