After searching for a while this is the solution I've come up with - A chin strap that attaches to the mask headgear. I used the bottom portion of a Halo chin strap which has Velcro for attaching it to the top portion. Overall it's about 18” long. With the Quattro, I attached it to the top straps by looping it through suspender clips and on the Quattro Fx I just looped it over the top straps. I found that attaching it to the top straps worked better than to the bottom straps.
Once the strap is in place and adjusted, it can be left in place and simply slid off and on the chin with the mask. I found that the strap only needs to be minimally tightened so that it begins to offer some resistance only when the jaw starts to drop lower than the normal mouth closed position.
I've been wearing it now for a couple of weeks and it's surprisingly comfortable. It stays in place, the tension is directly up so it doesn't pull the jaw back, it doesn't add another layer of straps around the head, and it only needs to be minimally tightened. I had reservations about the suspender clips, but it turns out I don't even feel them. Best of all, it has completely stopped the jaw dropping and mouth breathing.
I was lucky to have the Halo chin strap on hand but I can imagine all sorts of of strap material working.
Maybe something like this will work for you.
Jay
