its not negative its fact that’s the long and short of it
pretty simple really if you want something on a rigid then something along the lines of a fridge or car courtesy light - pressure switch on doors linked to light/buzzer in cab, job done. Bit of a PITA on an artic though as the feckwits can’t even standardise the layout of the suzi couplings & parking brakes 
German Shepard in the back, if your doors have been opened you’ll find the guy what opened then pinned to the floor crying in pain
I think that you should have a reversing type cameramounted at the top of the box trained on the door handles.This way you will have evidence of the tamperer,the police might even show an interest as most of the work has been done for them.
A plastic snap together seal is commonly used on back doors, to check if the driver, or anyone else has been poking about in there.
Cost must be minimal cos everyone uses them.