Kevin-

I didn't reinforce the inside, as the extruded aluminum sidewall of the trailer is pretty stiff and the round head bolts have a pretty large bearing surface, but then the design I used is more like the second example from Etrailer.com. It spreads the load pretty well from top to bottom. I think even the Harbor Freight model would work, but a lot depends on how heavy the spare is and whether it sticks way out, increasing the lever arm. Rough roads will also exaggerate the load, which is why I would prefer the Etrailer model.