Trigger locks.
Yep, I always have a couple of extra FA-10 forms on hand. I keep one in my briefcase and one in my range bag just in case.
The typical "Master" style trigger locks are easily defeated if you know how. I have had several occasions where folks have shown up at the range and said "Oh darn, I can't shoot today. I forgot the key to the trigger lock on my gun. (most recently three weeks ago at the skeet range. Shotgun in back of pickup truck=trigger lock)
I can pop them off in about 30 seconds with no harm to the firearm.
I'll be glad to show you in person, however I will NOT post how here.