For woods/hunting/packing/survey use, I strongly prefer Simply Rugged and have been using Simply Rugged holsters for a good while now. For EDC, I like Raven, but the SR is far more versatile and comfortable in the field for multiple days and over terrain, IMO. I have worked in the Alaska, North Idaho, Eastern Oregon/Washington backcountry for years and spend many days and nights outdoors with a handgun each year. Always with a backpack with hip and sternum straps, and sometimes with a long gun, as well.
A simply rugged Sourdough for revolvers or a Cuda (open) or Tribute (if you want a thumb break) for semi-autos is about as versatile as it gets. Order with IWB straps (and Chesty Puller straps if you want chest carry) and the same holster can ride OWB, IWB, cross-draw, and chest carry. The chest carry can be adjusted to several cants and positions from midline to underarm, and 45 degree to straight. In any position, SR are very quiet, which is great for hunting; and fast, which is great for defense.
FWIW, I recently got into G20s and spoke with Rob about retention for a Glock using a Cuda for chest carry. With my other weapons, I prefer strong side, even with a 25-30L pack w/straps. But for mountain survey where I won't run into recreational hikers or for a heavy pack, fishing in chest waders, canoeing, sitting on my ass all night at the fire,... I like to have the chest carry option. Rob used to carry a G20 in a Cuda on his chest while living/fishing in Alaska and recommends adding a tension screw, which he can do if you ask. If you plan on spending most of your time in the chest rig, they recommend the Tribute.
Either way, I imagine they will be able to accommodate your optic and barrel length (open bottom anyway) and retention preference with a phone call.
If you were going exclusively for chest carry, the Diamond D would be an excellent choice, though you're not and it does not have the versatility of the Simply Rugged holsters.
Simply Rugged also makes flap holsters if you really want one. If you're looking for maximum protection and are still drawn to a flap holster, the Wilderness Safepacker would probably be a good solution for you as long as the Government size (they have +1" and +2") works for the overall length - I think you would be able to get the RMR in there without too much fuss, but don't know for sure. It's big-ish, but made for Search and Rescue backcountry carry, and will get you and your weapon to the same place at the same time. For some weapons, the Safepacker may be a necessity, but the Glock is a Glock. If you're carrying the G20 for defense against predatory people or critters, the Simply Rugged or similar holster will probably be faster into action and still concealable if need be.
I know I will deeply offend some here, but the M12 holster, in my personal experience, has not been awesome. I used one as an infantry medic (read: running, humping, jumping, scrambling, crawling, etc.) for about ten years with an M9 before I permanently moved to a better solution.
Whether in training or downrange, I have drawn from the holster under stress, and it is not ideal. I have also, on several occasions, had an M12 vomit my 'secured' pistol on its own while running, scrambling, or exiting vehicles. I don't suck and I'm not new, and in my opinion, the holster (several units over the years) did indeed disappoint. If you go that route, I would suggest two things: a) don't; or b) use a lanyard. Lanyards suck for backcountry and if in a thicket, will probably assist your weapon out of the holster (even a flapped M12), but at least keep you attached to both the pistol and the tree.
I know Mainsail and others will disagree regarding the M12. My experience is based on more than a decade of using multiple units of the same holster in an infantry setting, and is my own. YMMV.