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I use the Hill People Gear Kit Bag. This picture makes it look more tacticool than it actually is but I don't have any shots of mine personally and my phone camera got lanced by my car keys.
Anyway, they have several configurations of it that range from small to large, some with PALS on the front. Multiple colors. I have the original which has three conpartments.
Basic rundown is it mounts on your chest, rearmost pocket holds a pistol and has velcro so you can secure whatever. You leave the zippers parted at a corner so you can 'rip' it open, grab pistol with your other hand, and draw.
The other two pockets are for your essentials so that even if you somehow lost your pack or had to ditch it for speed or whatever, you'd still have most of what you need.
I love mine, it goes hiking with me, rides around in my car, and goes to work with me. Really well made. The draw is easy and natural and nobody has to know you have a big scary gun with you.