Inspired by https://www.mynissanleaf.com/viewtopic.php?f=37&t=14081 and having needed a spare tire one time with our 2012 model, I got to working on how to carry a spare under the trunk. I had looked before under the 2012 and found the space constrained by the parking brake actuator. It appears that the 2013 and later models put it somewhere else making this empty space available for a spare, like in the way that the Australian cars have a steel basket carrier there. In my incident with the 2012 model I found that the compact spare from our 2001 Chrysler Voyager worked just fine to use the car around town until new tires came in. That van now has a full size spare making this 145/90/16 size available. It is a steel wheel so the Altima spare might be lighter if it's aluminum as claimed. This project all came down to making brackets.
This hanger is suspended from the trunk floor with four bolts and large washers to spread out the weight. It is five inches tall. Those are wheel studs sticking down so the spare tire uses the same size lug nuts as the car, so only the lug wrench is needed to dismount the spare. That 1.5" square aluminum tube bolted in forward is a spacer so that the tire is resting equally on it, the middle trunk stiffener, and the bottom of the back of the trunk.
Here you can see the bolt pattern in the trunk. One result is that the spare hangs about an inch aft from the UBUYGAS installation.
I cut the plastic air diffuser similar to UBUYGAS though at slightly different places because I also had installed a bike rack that required some close fitting. I used Lynch pins to hold it up, and for good measure added an aluminum stiffener. I used 3/16" aluminum rivets for the hinge and stiffener connections.
And there it is. Considering how much time it took to work all that out and fabricate the parts, it doesn't look like all that much in the end. I stole the jack and tools from my old decrepit Toyota. So now I have self rescue available in case the compressor and a plug kit are not enough.