I'm going to be building a bike rack in the back of a truck. I've got basically two options for fork mounts, Thule 821 or Delta Pro Hitch 2. The Delta can be locked, but realistically I'm going to need a cable lock anyway to keep someone from just lifting the 2x4 out of the truck. The only other nice thing about the lock would be that it would keep the skewer from accidentally opening.

Does anyone have experience with these fork mounts that can provide some opinion on their quality or ease of use?

I wouldn't worry one bit about the skewer accidentally opening. To my way of thinking all of the fork mounts are about the same so just buy whatever's cheapest. I used to make my own until the price of commercially manufactured ones got so low that I decided it wasn't worth the effort.

When I has my pick up I bought two keyed alike padlocls and ran a cable across the bed of the truck locking each end into a cleat. That's about a 90% security solution but it worked adequately for the 12 years that I owned the truck.