Piriformis stretch against wall ("Figure 4" Piriformis/gluteals stretch, supine, foot on wall; 01)

Lie on your back with your feet facing a wall.
Bend your unaffected leg and place your foot on the wall
Bend your affected leg and place the outside of your ankle just above your unaffected knee.
Allow your knee to relax outwards.
Move your body towards the wall until you feel a stretch in your buttock, and then hold this position.
It may become easier, so move yourself further in to the wall.