In order to work with a family guard dog in personal protection work you need a sturdy leather collar.These collars are relatively inexpensive, but need to be special ordered since a regular dog collar is not strong enough, and does not give an adequate margin of safety. If a collar breaks while working an aggressive dog, the person helping you is...well in a tight spot, so don't skimp on the collar.
Also, a good collar makes your dog look more "impressive" providing greater deterrence and de-escalation ability of the dog than without the collar.
I have included a picture below of a really well built leather agitation collar that I purchased from Leerburg.
Notice how the collar is made of good leather, is double stitched, and has a strong, curved metal piece to hold the leash snap.
Here is a more detailed view of the inside of this collar showing how well it is put together.
This collar will not break even if the dog is pulling like mad (which they do during protection training and real life encounters) and you get a real firm grip on the dog when used in conjunction with a leather leash.