I have seen folks use a few options to look the extender to the hitch. Ive seen hitches with two holes and one used as a lock with a locking pin. Others have used the locking pin to hold the extender. It has some play but isn't an issue. The most creative was a welded on deal with a master lock...
I can't say enough good things about the 1up racks. I wouldn't carry my bikes on anything else. Yes its expensive but worth every penny. It will hold any bike you can imagine. You can adjust how they sit left or right on the rack to avoid having them touch. We run into spare tire issues at work...