Join the NRA. Take the 8+ hour $100ish course and pass. You are then technically the lowest level of firearms instructor which may be enough to qualify you for a glock armorer course.
I'm taking the glock course in May. Signed up 6 months ago. They allowed me because I am a gunsmith and range officer. Basically they just want to know you have formal gun training for safety.
I don't need No stinking signature.