For my vent cover I use the plastic cover that came on a car wax can.
They are not all the same exact size ... and in this case ... size
matters.  I don't recall the brands of wax for the ones talked about
here.  I haven't tried Turtle Wax ... but that little turtle on the
plastic lid would seem appropriate ... if it'll fit.

I had one that I had to slit at one edge ... overlap it slightly then
put together with JB Weld.  That tiny overlap was at the bottom when
installed and couldn't be seen.  The problem is ... when I had it off
one day, my brother thought it was trash and threw it away.

The one I'm using now (and have been for 3 years ... including freeway
driving) is slightly smaller so I have a ring of 14 gauge electrical
wire that I put around its perimeter to achieve a snug fit.

I can take pictures if anyone is interested.

Mike