some thoughts about chinstraps

if you are mouth breathing with a nose only mask its unlikely any chinstrap will keep your lips sealed and prevent it. the chinstrap is intended to remind you to keep jaw closed but taping the lips is the only way I know of to ensure no mouth breathing or air leakage

a chinstrap can help with a full face mask to prevent 'opening wide' during sleep and so breaking the masks airseal

some make their own chinstraps from pantyhose, or a roll of elastic

with long hair, any strap around back of head should be under the hair and against the back of the neck for better results