Its true that a serious thief can get into about anything, though thats often given as a reason to do nothing, minimal security, or abandon whatever idea it was entirely (not saying that was the intent above). Im of the opinion that most crimes are committed by less than top level crooks, and making something more work than its worth, or even less conspicuous can deter or divert the majority of crooks and is well with the effort.
I needed a gen for my window AC for my dog so I could travel safely in the summer and be able to leave the dog in the truck. I modified a hitch mounted carrier to raise it up to reduce dragging on ditches and low spots and built a wood box. The folding adapter I used also allows me to tow a trailer with the box mounted, so I havent lost any capability in having it mounted. The only real down side is I camp open the rear hatch door, but I can access most of the junk I keep in the back area through the window or from inside the 4runner.
Ive used it for I think 4 or 5 summers, I mount it as soon as it gets above 65 in the daytime, thats my cutoff point to leave the dog in the vehicle without AC capability and only take it off when iot gets cooler than that in the fall. This is with semi-regular cross country trips and staying at motels, but I select morels where i can park right outside my door and have a window to see the rig. I sometimes set a camera in the window with the monitor next to the bed so i can see what the motel noises are without getting up. So far no problems with anyone getting in it. I do get a lot of people curious about it when they hear it running or see me start it up before going into a store or restaurant. The side benefit of all this was I could now take my dog on errands around town when not on the road when it had formerly been too hot to leave her in the vehicle.
Heres pictures and the general idea of the box.
https://bushcraftusa.com/forum/threads/full-hillbilly-window-ac-in-a-vehicle.228995/
I have details on how I modified the folding hitch adapter if anyone cares to see how it was done. It essentially raises the cargo basket about a foot or so and eliminates the ability to fold it, though it does allow the carried to tilt back in case I had to access the rear hatch door, but if it was loaded it would be a pain in the behind to do.