I always rinse and re-use mine. You have to be careful...too much use or too harsh of a rinsing and they will start to come apart. I have two Whisper 30/60 dual chamber filters, each chamber can take up to two cartridges. What I do is use one cartridge per chamber and put a cut piece of foam in the other slot as a "pre-filter". When it's time to clean, I squeeze out the sponge and rinse. Then I lighly rinse off the filter sock. Even using a total of 4 of these type cartridges at once, I haven't had to buy replacements since October.
To answer your question, I guess it's how creative you are. You could use a regular carbon sock and sort of flatten it out, but they are about the same price. You could sew up some material, but I really don't think it's worth it. About the only solid advice I can give you is to buy in bulk (buying a box of 6 socks is only 2 to 3 times as much as buying a single replacement cartridge).
A hint...these are even cheaper at Wal-Mart. It's the only fish related item I buy at Wal-Mart