We just used a 1/4 20 bolt. It held fine. The plate closest to the wheel was tapped, and 1/4inch thick. The other plate is not tapped. We only had to tighten the bolts once per competition: right before any final match. It's a wonderful setup. This variation is a bit heavy, due to two bearing blocks. I've since redesigned it to only need one frame rail, as well as only one bearing block.

This design, with two bearing blocks, yields a beastly solid frame, but at the cost of too much weight. The singe block design is much lighter, though less beefy.

Are you thinking of maybe using this design?