2 sponges and a layer of filter floss in between them will work great. That's what I have in an AC50 anyway; clean the floss often though, or it will cut the flow significantly when it gets really dirty. In my AC110, I just use the sponge and biomax that it came with, but I also use a sheet of filter floss which covers the top of the media and goes down over the waterfall part, so it silences the water outflow and works good for mech filtration.
I just checked mine, I have 5 inches between the tank rim and the wall. The filter takes up about 4.5 inches. A large powerhead with a prefilter works good for mechanical filtration as well and is also cheaper.