i do these stretches.
this one should really lengthen the piriformis. you don't feel anything with this one? i do this one, and usually gently try to massage the muscle (in general area, since i don't have really sensitive hands; i dunno exactly what i'm massaging) with my finger tips with light pressure. this seems to have a help. then i cup my hands and lightly tap the entire butt cheek area as well.
downward dog also seems works well for me, especially the one leg in air one too.
laying pigeon also works well, especially when you lean forward (could be hard if you're not flexible) but you can control the amount of stretch.
half spine twists. if you can stick your arm through the legs like the 2nd photo then you will also get really good stretch on the piriformis area (as well as many other muscles).