I don't know if it would fit your Glock 20, but I really like this one?
https://specgear.com
I personally prefer the right hand cross draw one.
https://i2.wp.com/specgear.com/wp-content/uploads/2013/03/cross.png?ssl=1
It does take some fitting to find the 'sweet spot'. I cut off a lot of the accessary stuff. Just leaving the holster and some spare mag pouches on the right hand side. It's extremely comfortable for me, once I get it properly adjusted. Once adjusted, it's pretty easy to put on without any problem. If your chest is right on the dividing point in chest sizes, order the larger size.
Of course with a concealed shoulder rig, you have to either pull a 'Superman'; or have shirts with snaps instead of buttons. If I'm using buttons, I frequently leave the second button down unbuttoned.
I have a drawer full of the traditional waist type holsters by many different makers, but with my bad back this is the only one I've ever found that is truly comfortable for all day wear. Again be prepared to spend some time adjusting it to find the sweet spot.
But since everything is velcro, fine tuning the adjustment is relatively easy. A mirror is handy for the adjusting process.
Edit I just found this in the FAQ section.
What firearms will fit in the TRUSS® Holster?
Almost all semi-autos and many revolvers will work fine in your TRUSS®. If you have a specific gun that you want confirmation, just email us and we will get you an answer. We also can do custom orders if your gun doesn’t fit in the standard TRUSS®.
Edit two. I personally carry my P365 up fairly close under my armpit, so the end of the barrel is above my belt line. Then I slightly blouse my shirt and carry a small glasses case in my shirt pocket to help hide the outline of the pistols butt. I also cross the shoulder straps on my backside, to keep them from slipping off my shoulders. And I don't use the pistol retention straps that come with it. If I've ever been 'made', no one has ever said anything.
At my age, I'm not engaging in hard physical activity. It is slightly warm in hot weather. But unless working on the farm, I'm generally in AC on summer days. For sweaty outside working on the farm, I have a conventional shoulder holster worn open carry style. I only conceal when I go to town.