I used ladies stockings to hold the Matrix in small (size of small orange) packets, and then placed several of these in each center basket of the FX5. You can fit 3-4 of them in each basket, depending on how big you make them, and the stockings allow water to flow freely through, without letting any of the Matrix out to possibly damage your impeller.
Stockings are cheap (free if your Mom has a few dozen extra laying around ), easy to use, and work well IMO. I haven't had ANY noticeable reduction in flow using them, and the FX5 keeps my 220g, heavily stocked tank clear and clean.