The Sienna did not come with a full-size spare tire, only a temporary spare tire. I wanted something more useful than a temporary spare, so I bought a full-size tire and rim for use as a spare. For the first few road trips I carried it in the cabin with all my travel gear, bolting it to a piece of plywood to create a shelf.

Eventually I tired of that arrangement and looked for another way to transport the spare. I wanted to place it in the seat well, but the well was designed to hold a flat (deflated) tire, and the fit was tight. A fully-inflated tire has a diameter almost 1.5 inches greater than that of a deflated tire. The answer was to deflate the spare and store it in that condition.
I bought an air compressor for inflating the spare.

The walls of the plywood box were in the way of the spare, so I removed them. I also had to alter the support structure of the well's lids, replacing the rear wall of the box with four posts, some L-brackets, and two boards atop the posts. One top board (left foreground in photo) is directly over the tire and must be removed to get the tire out. Four screws hold it in place.

A later modification on the right side of the box added more functionality.