Hey Henward,
You will be wanting to use an open cell structure foam that has not had any chemicals used on it. The density should be 10PPi for the first phase and then gradually denser all the way up to 30PPi as a polisher which has the added benefit of being an excellent host for the beneficial bacteria you want to encourage in your system. I use them in my breeding vats. Check out this website http://www.swisstropicals.com/Poret%20Filter%20Foam%20Pricelist.html I've ordered a large quantity of foam for my set up and might have a bit going spare which can be cut to size for you, a hole lot cheaper than the FX 5 ready made filter cartridges, and I might add in my opinion it will be better quality.
I've been keeping fish for 15 years and have always made my own filters but recently I started looking into the sponge filter, what can I say but I am totally sold, and not looking back. They are easy, effective and and great bang for your buck! Look at this article http://casb.okstate.edu/casb/fishchum/index.php/scientists-featured-articles/79-freshwater-filtration-novel-insights-and-practical-considerations
Cheers, Greg
P.S. a go pro underwater camera mounted in your tank at feeding time would be killer!