It appears to me that if you slide the cooler back a little you could weld a couple tabs on that shock crossbar. It wouldn't take much to support the other end off the spare mount. Possibly an "L" shape from the spare mount over to the main frame rail. It would still clear the spare tire.
I absolutely LOVE the clamshell holder, Carey gave me one of the first. Probably the only Vintage Spyder with one LOL!
Keep working Tom.