I too use the space under the back seat, spare H4's, breaker bar, assorted key wrench, shovel, axe....on and on...
Since I'm not a proponent of roof top tire carriers or bumper tire carriers, I secure my spare in the cargo area with a cargo net to keep it in place. I have found that I can stash all kinds of stuff through the center hole of the spare tire...rags, work gloves, WD 40, bungee cords, some assorte rope, etc.
Where the spare use to mount on the side, I found that couldn't mount my wheel well mounted subwoofer now that the spare is on the floor (not enough cable length). So I cut a similar sized cirlce out of masonite with an appropriate sized hole so the spare mounting rod could poke through, and then mounted the subwoofer in the upright spare location, only without the spare. This setup created a spool of sorts, and that's where I keep my pull strap secured. I've seen folks mount garden hose hanger there to keep pull straps there also...
Ivan