Philips Nasal cushion masks for CPAP and BIPAP therapy create a seal around the nose only.  If you can normally breathe through your nose OK, these mask generally are easier to wear than full face masks.  Sinus cleansing can help ensure free nasal breathing and chin straps can help keep the mouth closed.

Philips provide the Wisp Minimal contact Nasal Mask and the DreamWear Nasal Mask