I can't remember the last time that I had a flat tire, except for my tractor which the tires have age cracks and loose the air. Get a can of "Fix A Flat", and it will fit perfectly under the plywood. If you really want to carry a spare, get a space saver spare, and you will make it home on it. The only reason to buy a spare tire, is because should you damage a tire beyond repair, a spare tire will save you a lot of grief finding a replacement. Most tires that you buy today will be discontinued in a couple of years, and finding an exact match is next to impossible.