Hmmm, I wonder if you could make something simple to hold the tire upright? Like a pipe in a flange that is bolted to the floor in the back, with an elbow on it and another length of pipe to go through the center bore and then something on the other side to tighten it all down.
Did that make sense???
Edit: Or, what about a tube that you somehow bolt to either side of the VX, through the trim (basically a horizontal bar) and then you could bolt or strap the spare to that piece??
Bart