For awhile, I had the tire carrier removed from the back door, and I bolted it to the floor, in the back. That held the spare flat in the back, just fine. I put a piece of plywood and a blanket over it, for the dog to have a perch.
Today, I have the Calmini swing-arm, and my spare 33 rides there. I have a big plastic tub bolted into the back now, for tools and stuff, but I'm re-thinking that.