The other slit on the tank cover should be for a hob
thermometer but you can hack up the lid for your needs.
I'm going to put a biowheel 330 gph hob
on one side of my tank. The other I am mounting a h.o.t. magnum 250. Hopefully I can modify the tank output/input tubes so I won't hack up the lid too much.
I've never been into sponge filters but I've heard some good things. My wife likes the air-bubble lines (if you do this get a valve to throttle the air), which do cause more air turbulation but it lacks any cleaning ability.
Another option that also moves current but doesn't clean is a powerhead... not sure what the lowest gph
is tho. Certainly don't want to stress the fish by slamming it against the glass. 8O
You could probably make a sponge filter with a powerhead but not sure exactly how easy/cheap it would be compared to retail purchase.