I think vinyl it going to look a lot better than grip tape and will definitely last a lot longer. Make sure you get a heavy mil vinyl and one that is rated for at least 5 years outdoors.

 

I sail Hobie Cats and they make a heavy-duty neoprene material that glues to the siderails of the trampoline. It should work pretty well , as it is made to stand the weather and has a waffle pattern. Only comes in black, though.

 

Lol title made me laugh, but it might work