triangulated brace that goes back to the wall or back up to the underside of the work surface. Given you have three different heights the brace should telescope. Perhaps recycle one of those telescoping snow removal brooms for cars. They have that special plastic connection that telescopes and secures in whatever length you want when you twist (torque) the connection down. You could create a pivot brackets with a pin to pivot and adjust to the different angles.
If you do your math (measuring) right you could also use a telescoping strut channel with a pin to lock it in place for multiple positions
