Spare Tire size.

Not sure who is looking to do what here.
To fill the hole, I'd just make a block out of styrofoam or a frame out of wood.

Looking for a useable mini spare, then I agree with the above response.
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Find one with diameter close to the other tires, that meets the rim mounting range, and has a max load in the same ballpark or better than the originals for your car.