You can do it. But you will have to add stiff foam around the perimeter under the cover to get it level. At least if the firm backer board is full size to the the back cargo area. If not, you will need to cut one out using the carpet as a guide. You will loose some load space, and you will have an elevated flooring. Not to sure that you really would want to do that.
Maybe getting a skinny rim and tire is the answer if your wanting a real tire for a spare.
Thanks, all. I was just brainstorming an opportunity of circumstance.
This week I bought a spare (5th) OEM alloy rim on eBay. Couldn't pass it up at $55.
Of my 4 present tires, one is about 20Kmiles newer than the other three, as it was replaced under road hazard coverage. When the time comes for new tires, I'm thinking of buying a new set of 4 while keeping that newer tire on its rim as a spare replacement for the doughnut spare. Otherwise I may just store it at home, tire and all, continuing to use the doughnut as my spare.
Thanks for the tip on how to adjust the floor height should I decide to do this.