I got some random generic one. I wouldn't recommend it as the quality could've been better. For you, I also tried tape over the mouth. I got a wide washi tape from a friend and experienced similarly helpful results to the chin-strap. However, I found that comfort-wise, I preferred the chinstrap.

But, if you can breath through your nose well enough, I think some kind of mouth tape can work? I haven't tried the Hostage brand anti-snore tape, but maybe someone here has had experience with that.

If you really want the chinstrap that I got, I got this one: https://www.amazon.com/gp/product/B08DR16XGV/

Based on the pictures, it obviously pushes the jaw backwards. That obviously wouldn't work for you. I wear mine in such a way that it is helping my jaw close and stay shut. I agree that you don't want something pulling your lower jaw backwards.

Yep I've tried mouth tape and it's pretty good, especially when combined with side sleeping.

Though I have this paranoia that mouth tape will increase my philtrum length, which I know is a vain worry. Another solution is one of those incline pillows - just a 20% gradient should allow me to back sleep, which I prefer.

Post-surgery I had my jaws wired shut and they had this chin strap that was like an ace bandage that around my head and chin. I could replicate that with one of those mouth guard combined with a chin strap, but I read somewhere that this would eventually pull the upper jaw down towards the airway as well (since maxillary plates are not fixed.)

Have you tried mouth taping instead? Seems to work better for some people than chin straps, but I don't really know.