That was my first revision of the design!
I just crammed a load of biorings in the bottom and squeezed the sponge on top. This worked but wasn't ideal. The biorings lifted the sponge above the bottom openings in the sides of the filter, exposing the biorings to the water straight away. I noticed most of the water bypassed the sponge and went straight through the rings (path of least resistance..). This lead to loads more clogging in the rings, and generally more crud making it's way through to the impeller...
It's so annoying when you consider how good the U2, U3, and U4 are that the baby of the family just doesn't cut it!
Feel free to try a few configurations.. I found as long as there's nothing directly on or blocking the impeller inlet it'll work well.. Often after I've put the rings back in between the sponges I flip the media basket (lid on!) and poke about through the hole in the bottom to make sure the suction path is well clear..
As a bonus having two half sponges makes media changes safer, you only change one half-sponge at a time. Retains well over 50% of the bacterial colony so minimal risk to your stock. You can see in my pic the sponge on the left is more discoloured, left is 12 months old right is about 6 months old.