Kenai chest holster for me. I’ve used it a couple of seasons now during archery season and on scouting trips. Zero issues. Comfortable. Secure. Easily accessible.
I don't have a Glock 20 but I do carry a S&W 69 (44 mag) OR a Sig 226 for protection when hiking and hunting. I was in the market two years ago for a chest holster since I am commonly wearing a backpack and waist holsters don't generally work well. Previously I had a thigh carry rig which was fine, but not ideal. After researching the pros and cons of chest holders available at the time, I went with the Kenai Chest Holster. At the time I was looking, the other main chest holster option was a leather holster set-up. The Kenai Chest Holster fit my needs much better plus you can order multiple holsters (for different guns) that simply snap buckle into the harness (strap system). I appreciated this level of convertibility and the overall quality of the product is excellent. I believe there are additional carry options available now but the Kenai is the one I have personal experience with. Hope this helps...
Blackhawk Serpa for me as well. Carried it that way since 2012 when I bought it. It’s fit every major pack out there so far other than Kifaru just because I haven’t owned one.
Razco makes options that attach to a bino harness which I run when hiking in the off season as well. But other wise lots of kydex makers out there. Personally I have a safariland duty holster with a molle adapter to run on my pack belt (kifaru) it's a heavy full retention holster so I mainly run it for bow season.
I have always like kydex for a holster, handgun stays pretty secure without any additional retention. A good holster maker can make one for your preferred carry option.
Different holster retention methods create a problem when you wear different holsters on a day to day basis. Kydex without additional retention draw the same no matter where you wear them. Nothing quite like trying to draw your handgun and finding that you did not undo the retention. When a 1/2 second may be very important you want the same draw process for all your holsters..jmo.
I carry the Blackhawk Omega VI drop leg holster. The material is quiet when walking through brush since it isn't the kydex, and being a drop leg holster it doesn't interfere with a backpack waist strap or pull down your pants. Also like how when standing still or even walking it is always within 4 inches of my hand. Comes in black, a couple solid color options, and a camo option
Blackhawk serpa for me. I wear it on my belt and its rock solid and easy to draw. i have a somewhat low profile waist belt on my pack and there's no interference on the few times i've had to draw (thankfully not shoot yet). i also like the fact the gun will always be with me even when my packs off. I tried the chest holster bino rig and it was too much going on and not a natural draw for me. Security and speed of draw is the main reason i quit carrying my .44, I couldnt find a holster that seemed to do both.
Another vote for the Blackhawk serpa. I carry a small pack and it does not interfere with the holster. I carry my binos on my chest, so a chest holster will not work.