Step 1: Parts List
Whisper Bio Bag aquarium filter cartridge small size
9V Battery Connector
40mmx 40mmx 10mm computer fan
4 screws (I used m3 x 18mm screws I had lying around) and nuts
switch (I got this one out of an old clock)
Dremel with a cut off wheel
Step 2: Making the Case
Drill the screw holes first then cut the hole for the fan. I used the Dremel for this but a hole saw might be an easier way to do it. The extra copies of the layout came in handy for this as the paper tears up a lot faster than the tin.
For the lid I traced a rough outline of the filter with a sharpie and gradually cut it out with the Dremel. You will want to clamp it down or put it in a vice to hold it in place. If you get it close enough the filter will stay in place without needing anything to hold it. You want the filter to be as close to the opposite side of the hinges as possible to make sure it will close when everything is put together. Removing the lid from the tin first makes this step easier.
Cut the hole for your switch wherever you want to place it.
File down any rough or sharp edges
Step 3: Wiring
Solder the connections together as shown in the diagram. The circuit is pretty basic.