Thanks, I hadn't considered anything from the automotive side of things... "trim ring" may have been the search term I was needing.

In fact, since this is a home use situation, if I had a decent bezel that looked good and held the guns right, the "container" part of the holder could really just be a black cloth bag attached to the underside of the hole.
Edit: and I was sure there must be some sort of plastic end cap you could put on a 2x4 for putting it in the ground, etc... but I guess that is not a thing, unless my google-fu failed me.
