I would say to get a good watch dog and put him inside the building at night....
the cam idea is a good one and even a security company sign.
with cams, security sign and fido the ones looking for an easy score might move right on by.....
if you dad has some welding stuff I would probably make up a heavy wire cage to roll toolboxes into at night.
the more layers of defense you can add to a place adds to delays for Mr. Criminal.
Have a nice day at the range
NRA Life Member