I would do away with the foam and just have a load of filter floss. It will clog up quicker, but you will only have one thing to rinse (I rinse mine at least once, often twice per week, but it really pulls stuff out of the water) and will have more room for bioballs.
I wouldn't use the overflow, so long as you maintain the thing once a week and the chamber for the filter floss is at least 1.5-2" deep you will never have it overflow. The aim of the filter floss is to ensure that as little mechanical waste as possible hits the bio-balls.