I finally had enough of the lovely aires wafting from the vent every time we pushed the old plunger down. Had looked at a couple of commercially available vent filters but balked at the price. The OCDIYD disorder kicked in again and the result is this home made device:

Activated carbon is amazing stuff. Sponges up any hydrocarbon like crazy. An internet search found this. Evidently the tropical fish aquarium people use it since the hits were for their suppliers. This was about $6 including shipping from Amazon.

Some fittings from Home Depot and a 10" piece of 1 1/2" pipe. The end nipples are some 1/2" PVC I turned down on a table saw jig. Could have just as well used hose barb adaptors but I already had the 1/2" pipe and saw jig.

Some bronze screen left over from some douglas fir screen doors I made for the house last year. Fiberglass screen would have worked as well.
Put on one end, filled it with the carbon and glued on the other.

Here it is installed. I would have gone bigger on the length or diameter but this was all the room I had.

The initial tests are completely successful. Not a whiff. We will see about the longevity. I made up 2 of these while I was at it so when this one stops doing it's thing I will put in the replacement. The container of carbon has enough to make 2 more. Wrapped the spare tightly in Glad Wrap so it wouldn't pick up HCs from the ambient air. Total cost for each unit is about $12.00.