Nv Guy, I fully realize the reality of "no 100% solution" as all security protocols are defeatable given enough determination and time. Unfortunately, I am also familiar with my adversary because I have been hit before and this yard has been hit before (as have most yards as you pointed out). When I first got my trailer they broke the door latch lock (and I did not know I had not locked the deadbolt) so they took my TV, a small inverter, a backpack, 2 rolled up sleeping bags (but not the unrolled ones) and one of 2 propane tanks (the full one). Now I use only the deadbolts, because I learned how easy the latch lock is to break and they break them to see if you locked the deadbolt. They also stole one of my 2 6 volt batteries, even though it was locked they were able to jimmy the bracket loose. I have upgraded all of those previously hit areas and I no longer leave the TV in the unit (and I don't roll up the sleeping bags, lol).
I bring all this up because these are apparently vagrants and looking for supplies, not so much resale value. So I believe that if I can harden the target they will be more likely to move on. If tweakers show up, well then there will be damage I guess, but homeless losers tend to be less rabid. My question here is to see if anyone has hardened these particular compartments to deter theft. The L channel solution is a front runner. I also have my own idea but implementation might be difficult. I will post my results. I'm not trying to be boorish, but telling me to give up or move are not particularly helpful. I am aware of my options in that regard.
PS The storage yard has also made efforts to upgrade their security, with a night patrol and additional lighting and cameras. So at least they aren't indifferent and realize they get a rep in the underworld of thieves that could cost them customers.