I had this same problem because I’m very thin (6’4, 175 pounds) and have no butt to hold the saddle in place, so every couple of hours I found myself pulling it down back to where it was comfortable but it would eventually ride up again.
I fixed this issue by taking a small piece of my old Thermarest Z-Lite sleeping pad and sliding it inside the leg loops between my butt and the lower part of the saddle. It adds comfort, keeps the saddle in place, and takes up the space that was loose before. This allows the micro fit adjusters to do their job much more effectively and essentially shortens the bottom strap like you mentioned by filling the space. Makes the Mantis feel more like what I imagine the Phantom feels like with the “cupping” design.
I carry this pad In on my pack and it serves to keep my sticks and platform separated and quiet when hiking in to a spot, then slide it into my saddle before climbing.
An added benefit in the late season is this adds some insulation to your back side, obviously this could be a negative on early season, warm weather hunts but i haven’t noticed it being an issue. It’s certainly Nothing fancy, but it works wonders for me.
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