I made mine out of PVC pipe.
You will need about 18 inches of pipe, 1.5 or 2 inches is good.
A PVC threaded coupling (so you can take it apart in two pieces
End pieces with a barb fitting to connect your vent hose, usually 1/2 inch or 5/8.
Activated Charcoal from a local pets store.
Once you've assembled the end caps, the barbed fittings, and the pipe, cut out circles out of the scotchbrite pad as follows
- the diameter so you can place one at each end of the inside of the pipe, keeps the charcoal from going out the barb fitting
- the diameter, a bit snug, to hold the charcoal in place while you screw the two pieces of pipe together.
I replace the charcoal and scotchbrite each season, costs less than $5.
The pipe and fittings will probably cost about $20 or so.
Replace the vent hose and clamps while you're at is may be a good idea.
Just cut out a piece of the vent host, place your pipe filter inline, tighten with clamps, and secure it to floor, hull, etc.
Idea comes from this url => DIY Holding Tank Vent Filter $cheap