Thanks! What I ended up doing was similar to your suggestion. First, I put on a nut and screwed it all the way down, then added an extra-large washer with another nut tightened against the washer. Once everything was snug, I put the bolt in a vise with just the washer sitting on top of the jaws. A few smacks with a hammer starting bending the washer on both sides. I finished the bend on an anvil. So essentially I bent the washer sides up 90 degrees. Hopefully this protects against slipping.