I also have an Aluma trailer, and came up with a unique storage system for my spare tire.
I first put rigid foam in the side walls, covering it with marine grade tight loop carpet. For the bottom, I laid some heavy duty rubber cupboard material.
Using 1" x 6" boards, I ripped them to the height of the spare tire lying down. 3 separate compartments were created, 1 for the tire and two for tools and any other accessory I thought necessary for road trips. Basically an upside down "T" which places the tire at the front of the trailer floor. The form lays loose in the trailer, as it mates with each of the four walls to discourage movement.
A quarter inch piece of plywood went over the top of this form, with rigid foam and marine grade carpeting as the final layer. On the aft portion of the plywood, I made two holes which allows my fingers to pull the floor board up with ease. A similar access port was made in the rigid foam/carpet.
I lost perhaps 5" of floor space height, but the plus side is that I now have a flat surface to put travel items and not worry about the tire rubbing against them.
This was a fun little project taking most of one morning to complete.
On an aside, I found a jig saw knife blade cut through the rigid foam board with ease and little to no mess.
- David