I have friends who have made their own “lockable” system for their generators, mainly for when on their service/welding trucks. While not full proof it does keep honest thieves (or atleast ones that don’t have proper tools or time) from stealing them. They use a piece of steel flat stock for the top plate, drill two holes for the side posts and place rebar through them. Then weld the rebar to the top plate. They put the plate over top of their generator and wherever they put their generator they secure it. (One has theirs at back of his cab, and drilled holes through his bed. He then welded a plate underneath to the rebar) You could do something like this to the storage plate of your trailer.
PITA, yes! Pretty effective, yep!