Horse stance but with a stick on top of your thighs (you can use a broom stick) that forces you to maintain a certain level of depth otherwise the stick rolls down.
I held a motivational month for the horse previously on r/flexibility here if anyone wants more info:
@etant Old kung fu movies will have good ideas. Half of training seems to be sitting in horse stance for hours and hours, usually balancing jugs full of water on the stick
@ace111 While a wizened sensei mercilessly shouts "Deeper!" as you cry and struggle to maintain horse stance with a cup of water balanced on your head.
@etant Oh damn, reminds me of an exercise I have to do every once in a while where you hold a plank, elbow a medicine ball, roll it to the other side, elbow with the other arm, and repeat until dead.