I was looking closely at the optional vertical mount on one of the Hummers in Phoenix last week. It appears to me it would be possible to use the vertical mount, with some modifications to lay it horizontal. Looking at these photos, you can see there is a hinge to swing the hub bolt away from the vertical mount to remove the tire. I am imagining laying that flat and bolting it from the other side of the tire. It also appears the vertical bar on the left side of the mount can be removed separately, see the attachment bolts at the bottom, and the split between the two sides. So remove the left side, and lay the hinged part flat.
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