Because I've got a radiator up front, I've put my space saver spare tire upright. I tied it to the strut brace with a luggage strap:
I've tie-wrapped some rubber pieces to the strut brace where the wheel touches the brace. I can still move it a little bit if I apply some force. It's probably not tight enough for racing.
It's quite amazing how much space there is in the middle, I can put the wheel pretty much upright and the bonnet will still close without touching the wheel.
Here I've turned it around and put it even more upright to gain some more luggage space: