I have this water fountain for my cats that gives them moving water to drink and it takes these filters. Which at the local (major chain) pet store cost $10 for a set of three. They have to be changed monthly and while $3 a month isn't bad for a filter, it still adds up over time. I discovered that they are held together with pins and aren't glued shut. So when I looked inside I found a material much like a green scrub pad on the top and bottom and activated charcoal in between. I knew that the green scrub pads were cheap, so the question became how much for the activated charcoal. I found a Salt Water Aquarium supply store locally that sells it by the pound. The bag pictured weighs about 3-4 pounds and cost about $6. At that point it was obvious that it would be much cheaper to clean and reuse the cases from the original filters than to continue buying them.
Basically take the old case apart, save one of the external filters for use as a template to cut out your new one. Empty and clean the filter case. It's probably gross and slimy from sitting in water for a month.
Use the old external filter as a template on your green scrubby. I used standard size green scrubbys and was able to get both of the pieces I needed out of one scrubby.
I recommend smashing the charcoal a bit. the original pieces were kind of big and would probably have left gaps.
Now just reassemble it the opposite of how it came apart.
Just like any Activated Charcoal filter, make sure to rinse well before installing it in your fountain.