Vinyl End Cap for rod/tubes [USA] Rubber End Cap
Above option is the internal rod diameter size range that Vinyl end cap able to expand for suitable usage.
Vinyl End Caps (VEC) are best known for protecting the ends of your spars, end caps also save wear and tear on your sail fabric from spar abrasion.
Vinyl End Caps can be used to slip over wing tip nocks for protection and to prevent the flying line from getting snagged.
There are so many sizes to choose from you can find a multitude of uses for them.
Try for a snug fit so that you won't have to glue them on. Do this by choosing a size slightly smaller than the diameter of your spar.