How about having Filled Tanks in Cammo for the area they sit in so they are blending with the Terrain say 5 feet from the Target Practice Tanks. That would give someone a nice welcome for Trespassing on the Property. Maybe attach some nails, bolts, 2-3” triangular pieces of 1/4” steel plate, ball bearings, lengths of 3/8” steel cable and such. A Hill Billy Claymore more or less.