I use shrink tube just like that on the old mounts. A peice goes between clamp and the hangar on the hangar faces opposite from the motors.
It seems they wouldn’t really be needed anymore with these new clamps, but they couldn’t hurt to cut long ways to open up the tube, wrap the shrink around the hangar where you are going to put the clamps so each tube opens up to line up with two faces on the hangar matching half a clamp each and clamp down over the shrink tube. They might stay on tighter this way due to the resistance of them pushing out on the clamp instead of bare metal to metal, and even provide a bit of damping for the vibrations through the mounts to the motors because of this small bit of extra resistance.
I can’t imagine these are to shrink onto the screws… Just use some blue loctite and let it cure after torquing down the clamps… Assuming these clamps are threaded.
Also, props to @torqueboards for great customer service following up like this with customers having issues with products.