No experience with closed cell foam on slings, but environmental concerns will make you have to retrieve it when you launch.
Have seen news paper used, which then is left to break down in the water
and also large plastic garbage bags, which get taped to the travel lift
slings, and the boatyard workers dispose of them.
If you want to use the foam, maybe you can tape it to the slings, and the workers dispose of it for you after launch?