Dave
I've got the same cradles, I made a small slit in the carpet to get the head of the bolt so that it sits down in the recessed area in the middle of the cradle, drill a hole in the cradle, and through the tube. Bolt thru the cradle and tube, you could add some 5200 and make it really solid.

Glue something rubberry, like a piece of radiator hose on the end of the bolt that sticks out of the tube with silicon, the cradles will hang down when they are not in use, and every once and a while they will spin to the hang down position when you are putting the boat on it and you dont want to put the boat down on the end of the bolt, it makes a nice neat hole. Yes, its been done before.