Simple plate like the one above but using a Free Air 10" driver (I believe the volume of the enclosure is too small for the driver being talked about - my first gen sub was an 8" solobaric in a 'glass enclosure).
Plate seals against the spare and gives some rear view thumping bass! Sure,it ain't no 12" in a sealed enclosure like I am running now, but when you need more space than bass, this is the ticket. I use the one in my rear door when I need to carry more gear or switch to the my 12" square driver in a custom enclosure when I really want to thump!
I can still do the plates and grills for $199 delivered - you add the sub since there is no use shipping a common item like that.