I have built some shoes to retrofit my electronics lab bench with castors. They are a cantilever design that should require no modification to the existing table feet. They lock into the existing feet. When the castor is swivelled to the rear, the shoe is pushed up and kept in place by the end of the shoe. When the castor is swivelled to the front, the vee part that locks into the existing desk feet take the force and prevent it twisting. Left to right is kept under control by having a tight fit on the square tube of the desk.