Oh, I didn't realize you were trying to maintain the use of a spare holder. Still, the offer stands. I drafted up a quick example of what I could do to meet your needs...
It might be a good idea to affix the OEM wheel retainer to the new mount with some gasket maker, if the compressor dimensions allow for clearance at the gate cover. You may also have the option of mounting the spare in typical fashion and then mounting the compressor on top of that with a wingnut for easy removal... maybe.