I did the under-side mod this weekend as well. Made up a backing plate to help spread the load on the inside of the trailer. I mounted mine just behind the axle to avoid interfering with the surge brake mechanism. 4 grade 8 bolts poking down through the floor locked in place with jam nuts and fender washers on the bottom. Rim goes through the lug holes and held in place with 4 more nylock nuts with washers. Bought an aftermarket spare tire cover to keep everything from getting too messy - we'll see how/if that hold up. While I don't love the idea of crawling under the trailer, I figure that the flat tire can temporarily sit in the front of the trailer until we get to a convenient location to repair and put everything back where it belongs. In 25 years of trailering gliders I've only had 2 flats while on the road (maybe I've been lucky?), so hopefully it's a rare event.