Custom shotgun rod holder issues

Most boats utilizes forward mounted rod holders. These are usually bent to the contour of the bow. That bend creates tensioned contact points with the rod when inserted. those points prevent the rod from moving around, rattling and making noise when mounted. Shot gun rod holders are typically straight thus allowing the rod to flop around inside the holder and make noise. What methods have you guys seen in shotgun style setups to prevent this? I've actually NEVER paid any attention to this before deciding to build. Thanks