factory sport
Ya know, I've kind of had a thing for Glocks the past 10 yrs or so, and my G17, 20, and 34 all work and play in the same G20/21 factory sport holster. About 15 bucks will get you one, and they are tough, reasonably secure, conceal well with a modest cover garment, and affordable. There is no mechanical retention short of a friction tab on the trigger guard though, and if you want mechanical retention, the Glock Sport won't satisfy.
Another concealment holster I've been working (though not with glocks) with a bit is the Safariland with the auto lock feature. There is a mechanical lock that is lever operated, very subtle, pretty fast, and more secure than a friction lock. It offers a level of retention beyond the Glock sport, at about 2x the cost. Running the little tab with the shooting thumb is about like tripping a thumbreak, and I am growing fond of the arrangement, having used thumb break duty holsters for a long time.