I was diagnosed c.2006 but have never used a machine. At the time my physios were working on turning mouth breathers like me into nose breathers, & recommended a small piece of micropore tape vertically across one side of the mouth to keep it closed when sleeping. At the time I was also suffering from reflux, so could also breathe out of the other side of my mouth if I woke with an attack.

I fold a little hem each end of the tape to make it easier to remove.

by-the-by, 2 of my folkie singer & musician friends built the prototype of Resmed's original machine at Sydney University way back in the early 80s. ResMed story