I realize that I didnt properly describe my setup-- I am not using the inserts at all, fattening up the rails so that the clips fit snugly onto them, specifically because I didnt want to deal with losing an insert and to nearly completely eliminate any play/clearance between the rail and mount. Like the other fellow, I used rubber hose tubing sliced and ziptied / taped on . It was fiddly getting it to the right thickness with trial and error, but once done properly, they have pretty much stayed on for years now, with occasional new tape going on top as old tape gets mushed a bit. I bought some hosing at a hardware store, they come in various diameters so I think had to return and get a different size as the first one was too thick. I used tiny zipties to hold them on, but over time the zipties moved, so tape worked.
I saw this tip years ago and seemed the best way to try to make life easier on the mounts. Any play or looseness is just going to put way more forces into the mounts with every bounce.