Here's what I'm planning on doing:
The idea is to make 4 sets of these to fit on the bottom of a mille migla stainless boot rack, with the plates being bolted through the tub.
The outer sleeve is kept in place by the inner plug, bolted to the rack with M6 screws, and it will hand twist into place on the base plate (the outer sleeve will be knurled). That will make the boot rack easily removable, so I can get to the spare tyre.