I carried mine up there for years, I just used one 6"-8" long bolt with some washers and nylock nuts on it so it would rattle loose on its own. Just put a fender washer on the bolt (or a wider plate of some kind and maneuver it up through the mesh and through one of the lug nut holes. Put on another washer and crank down the nylock nut on top.
It was a b!tch getting it up and down, it didn't drive any different that I could tell, I could never use a parking garage which was a pita, but the worst part was every Ricard and his brother had to make a fricking comment about my d@mn tire on top. It was literally to the point of ridiculousness how many people talking about my spare. You would think no one ever saw a spare tire before.