I've experimented with a couple arrangements... some of my Penguins and Emperors have only a 'wall' of foam' that fills the front half of the filter well. I was trying to recreate the flow you get when you use the blue bonded cartridges: the water flows into the back of the well, thru the media and out over the spillway. This seems to work OK. For the Penguins, I cut an AC 300 foam lenthwise, then cut to length to fit snugly in the filter well; I lay two pieces in, horizontally, one atop the other. For Emps, I did the same thing with an AC 500 foam.
I've since tried simply filling the media well with a foam block or two. For example, in the media well of an Emp, I'll cut an AC500 block so it's long enough to fit into the well snugly on both ends. (I've stopped cutting the blocks in half lengthwise.) I'll stack two blocks in the well. They pretty much fill it up and do a great job. For Penguins, I'll do the same thing, but with an AC300 foam.
The latter method is easier -- I don't have to cut the blocks in half lengthwise, and they tend to fit in the media well more snugly. They capture quite a lot of crud, so I know they're doing the job.
I hope this makes sense... if not, let me know and I'll give it another try.
Jim