Here's an example for the Losmandy GM-8.

These are made of Delrin (both the insert and the foot), which is a very hard plastic.  The foot is held on by a 3/8"-16 bolt (which is recessed into the bottom of the foot), and a #6 screw to prevent rotation around the bolt.

You will need a small amount of silicone sealant to make them stay in the legs.  (I would not encourage use of a glue--that makes them stay just a little too well.)  They add about 1/2" to the height of your tripod.

Also available for the G-11, and again, they add about 1/2" to the height of your tripod. Unlike the set for the GM-8, these are held in position by an internal screw.