This article discusses the legality and the practicality of carrying concealed without a holster. Incognito Concealment provides every answer for you query!
Although the Glock 17 is a full-size pistol intended for open carrying, the right choices can help you carry it concealed. Learn more here.

Incognito Concealment Custom Holsters

Is it legal to carry without a holster? - Quora

Walking the dog holster?

Do Federal Air Marshals choose how they can conceal their weapon/credentials? For instance, can they have a well concealed gun holster? Also, do they have the option to carry radio w/ them