Our Hymers stow their spare wheel vertically in the front locker, here it is fixed simply on a low profile "stool" that is through bolted to the front bulkhead. Secured to that by a big cone shaped hand nut to a stud welded to the stool.
The wheel's weight all resting on the checker plate locker floor, the stool only holding it back.
Those with a bit of DIY aptitude and the tools could easily replicate what this does.
Here I am thinking of a block of wood, coach bolted as our stool to the bulkhead, featuring a 12 mm stud sticking out.
Then a turned and bored wood conical keeper slipped onto that holding the wheel back, with a nut on the stud.
Note, the "stool's" standoff is much shallower, if as Hymer, the wheel's outside faces the bulkhead, simply because of the rims massive "offset", in reality only as "thick" as the tyre sidewall stands off the rim centre.
There is a downside, you can't either check let alone inflate the tyre in situ; but in real life our only lose a couple of psi year to year, when I remove it during my service.
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Back in the eighties when we last had an Al-ko underslung carrier, I soon twigged the need to "properly" service that, including using a rust inhibiting oil on the screw threads. Placed under the van it inevitably gets all the abuse the road can throw at it, though that's so easily countered by proper servicing practices addressing what it has to face. Ours never was any real problem, but I recall it helped very much once in France to have run the punctured wheel way up on our blocks, significantly increasing the access space to deploy the carrier and with it the use of the jack for the actual wheel change over. Then rerun up the blocks to far easier refit the carrier in place. Use what we carry like this can make life a lot easier, than struggling without doing so!