Get a mig welder with stainless wire, many will run off of a 5 K generator, put the ground clamp on the bolt head on the outside of the panel and then touch the tip to the end of the thread under the panel. That effectively makes it so the nuts will not come off as the thread is destroyed but the nuts can be loosened a bit. It would be a bitch to get them off but loosen the nut, slide a piece of tin with a slot in it behind the bolt head and cut the bolt head off from the face of the panel with a battery powered zip wheel. Few folks would have these tools in their back pocket. Of course, they could get pissed off and damage the panel with a rock.
BTW, engrave an identifiable number on each panel frame, stencil the back side. Make it obvious and highlight with paint. Even low class folks usually realize that stolen property with an ID number is a bad thing to have in one's possession. Odds are they will move on to the next item (or throw a rock at it).
The state of Maine just passed a law that all catalytic converters have to be engraved with the VIN, Maine has yearly inspections so if the VIN is not engraved on it, the inspection station has to do so. Anyone on the scrap side has to have the VIN on every cat, if caught with unmarked cat its presumed stolen. With the exception of driving over the border to NH, it makes them worthless.