A 3 inch pvc "drainage" cap fits over my vent pipe cap holder nice and tight,pvc drainage caps are not schedule 40 (thinner) found in drainage pipe section of the plumbing dept.
I bought 2 at Lowes for about $1.50 each,I did drill a hole in each cap and used the existing screw which held the oem cover/cap in place.Put silicon sealer over the screw and on all 3 legs where the cap sits on the cover/cap holder and its done,much more durable than the oem cover.
I did have to shorten the screw about 3/4 inch and re-sharpen the tip so it wouldn't be too long.Took me about 30 minutes to complete and alot cheaper than a thin oem cap or cap assembly.