Removing Rear Under-Car Panel

In preparation for adding a bolt to hold down the spare in the back I will need to open that big plastic panel underneath the car. Anything I should know before doing that?

You don't need to go under the car. I removed the rubber drain plug under the mat where the spare was to go, drilled a hole in it, then used a section of drilled strap steel, folded once for extra strength, and placed on a "J bolt" with just enough slant to get it into the plug hole. I then used another piece of strap on top (with the rubber plug on top), with another nut to hold them together, tightened them together, and voila! A hold-down for the spare. I never went under the car. Be careful not to drop the setup before it's tightened in place, though, or your original question will apply.

That sounds good, I think I'll try something like that.

Right now I have it strapped down using the hold down designed for the 110 V EVSE.

My spare tire just sits in the bottom of the trunk, in the cardboard box it was shipped in. There's not a lot of room for it to slide around, with the electric jack, electric wrench, portable 12v charger, and spare EVSE back there. I also added the cargo cover so items in the trunk are not visible, and it gives me a shelf to put the window shade on while driving.

As was pointed out to me before I anchored mine, in a crash *everything* not fastened down inside the car can become a projectile, and that includes the spare tire.