I haven't installed a lock but I made provisions for one by cutting matching longitudinal slots in the upper and lower angles (and UHMW tape) before I riveted them to the fuselage. If I decide to install a lock I will cut a matching slot in the latch itself, then install a cam lock so the arm pivots into the slot through all three "layers". I made the slots toward the front of the assembly, between the rod attachment and the catch bolt where the latch is widest.
That being said, there isn't any lock that will keep out certain individuals that are determined to get inside for whatever reason. The issue then becomes how much damage they will do to the ariplane in the process of breaking in. I think the damage potential is a bit higher with my described setup than say, the other setup with the lock outside. Just wanted to make a full disclosure, and in fact I'm still on the fence myself as indicated at the very beginning of my post.
Jim