Stretch for Adductor Magnus ( instead of hamstrings)

When I practice hip-hinging, and when I perform the recommended hamstring stretches, it seems that my tightness is higher up the leg (Right under the sitz bones).  I think I've identified the problem to be in my adductor magnus muscle.  The only thing I've found that works without making matters worse is to sit on the edge of a chair and apply pressure to the grove below my buttocks for self massage (rotating my foot in/out and/or sliding my buttocks away from and towards the chair edge). Can you recommend a good stretch for isolating this part of the leg/hip?