Bonnet nut is ≈44 mm diameter 16 mm tall cylinder with very thin walls and threads on the inside that holds in place a water mixing cartridge in my shower. It is supposed to be hand tightened and hand removed when replacing gaskets. Mine was in place for about 10 years and got stuck. Different shower models have hexagonal outer outline which allows use of a special wrench in this situation, but mine is completely round.
I made a plastic tool (pliers for scale) to wrap around the nut and then grab, squeeze and turn with a large wrench. It did not work right away, no matter how hard I squeezed it would slip as I tried to rotate it. I applied WD-40 generously and few hours later tried again – this time it worked. Not sure how much contribution was from my special part, but in case I need it again 10 years later, here is the drawing.