I didn't want to buy a castle nut wrench so I made one. A 3D printed wrench alone wouldn't be that strong, so I took a hybrid approach and made the body out of plastic and used M3 screws to grab the nut. This might not remove a staked nut, but it is cheap to print and doesn't take long. It requires 3 M3 x 10 screws. Feel free to modify this if it doesn't work for you.