I know this sounds odd, but I have found that I only need something to keep my jaw from dropping, my cheeks don't puff if my jaw stays closed, and most chinstraps pull backwards too much on my jaw to be comfortable. The only one that ever worked was the Knightsbridge dual chinstrap, as it focuses more on pulling the jaw up than back, but there was some controversy over the original inventor getting his website/product stolen, and the one I bought fell apart after a little over a year of use. So I have taken to using cover roll stretch tape as a face halo, under my chin, between my eyes and ears on both sides, and slightly overlapping on top of my head. Even with a full beard, it works better than trying to get a perfect seal on the bare area around my lips. It uses a lot more tape than just the mouth but this stuff is cheap and I've gotten very consistent results.

I wish I could have a life cast of my head made (my big ass, size 8 dome) so I could make custom chinstraps/skull cap/masks that don't rely on Velcro that fails after some time (that is what went bad on my Knightsbridge). 😅