Hi Tamid some years ago I made some security rods for a gun importer to be used at an arms exhibition. They were not complicated at all and extremely effective versus their cost to make.
The device for each gun consisted of a metal plug in the shape of the correct size cartridge with a rod a little over the barrels length firmly anchored in the plug. At the muzzle the rod was threaded and went through a hole in a metal muzzle support bracket. The other side of the support bracket was a tapered nut with a couple of holes to accept pins from a removable tightening key with two spanner flats. When the nut was tightened firmly and the key removed it was impossible to grip the nut.
Just to make things a little harder I used a left hand thread as a brain teaser and case hardened the tapered nut so there was no chance of pliers griping it they would always slipped off, and when you have the correct length of rod for the barrels length you cut the surplus off just short of it exiting the nut so the rod could not be griped either.
I hope the drawing is of some help not the best I know but I did it rather quickly.
