Lenore at Evil Mad Scientist writes:

This trombone cozy was necessitated by a skin allergy to certain metals. It is a rectangle of soft cotton material with velcro sewn on so that it wraps snugly around the portion of the horn that comes in contact with the trombonist’s neck. Problem solved– no more neck irritation makes for a much happier band member.

Allergic to your instrument? Trombone cozy – Link.