This weekend I set about trying to figure out a way to stiffen the trunk or hatch floor, mostly above the spare tire. My subwoofer was causing the carpet to sag slightly into the center of the spare tire, and I had put a small tear in the carpet when I accidentally kneeled down in the middle of the spare tire. The simplest solution ended up being a 15.5" diameter circular piece of 1/2" thick plastic. It sits right on the upper rim of the spare tire's wheel, and sits level with the top of the tire. You could probably go up to 15.75" for a tighter fit, but this seems to work fine. I had a local plastic shop cut one out of extruded polystyrene. It's very strong, but isn't brittle, so I don't expect it to ever break. Having something that sits almost on top of the wheel also leaves room inside the wheel for other stuff. I have some spare bulbs, the towing eye, and a few other random things in a round Griot's Garage case that's meant to sit inside a spare tire.