As you noticed, your chin will naturally relax after you fall asleep. I use something like this https://www.amazon.com/Sunset-4332439131-Deluxe-Chinstrap-CS004L/dp/B00PM53XR0 not the front and back just the oval part to keep my jaw closed. Furthermore... if you can keep your mouth closed like this you could just use a nasal or nasal pillow mask. They are a lot less bulky with the straps and stuff.