I assume that you've come to a solution already, but in case you haven't, another idea.
Getting the spare in and out of its home is something I've spent a ton of time thinking about - as others have mentioned, it never happens in a safe or convenient spot so time spent at the front end making the process straightforward pays dividends in use.
For trucks with angled boxes like yours, this solution would require some additional fabrication and complexity, but a swing
down tire carrier would solve a lot issues. You install the tire on its carrier mounts while it is on the ground and a hand or electric winch could be added to allow easier loading and unloading. It just rolls on and off by a single user.
This is a commercially made example by Genright - it is "designed" for various model year Jeeps, but based on how it is mounted, it is fairly universal, especially for trucks with vertical walls on the rear.
Shop for Jeep Wrangler rear tire carriers. Get the TJ/LJ Swing Down Rear Tire Carrier - Steel at GenRight.com. American Made Off Road Jeep Parts
genright.com
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