I know it seems ridiculous to spend $50-100 on a piece of injection molded plastic, but this thing[0] has been the best I've found for getting deep enough into my abdomen to loosen my psoas.
I simulate that with a lacrosse ball and a kettle-bell. I lay on my back, and put the ball about 2 inches to the outside of my belly button and then place the kettle-bell atop the ball. From there, I move it slowly down passed my hip/groin. Don't go too heavy on the kettle-bell. I don't go over 35# usually.
[0] https://pso-rite.com/