Because the wingtips are made of thin-walled fiberglass layups, they're pretty flimsy to the touch. I used some 1/3" thick pink styrofoam sheet from the Home Depot Aviation Supply house cut into 3" wide slats to serve as the lightweight foundation of fiberglass reinforcements.
First, I sanded down the surface coatings inside the top and bottom walls of each wingtip, then cleaned that area with acetone, then rough sanded again. I did this to be sure the new fiberglass epoxy would adhere.
I then glued the slats into place with 3M spray contact glue. I then applied 2-3 layups of 6 oz fiberglass cloth directly on and around the slats, making sure they extended 2-3 inches beyond the sides of the foam slat into the wingtip. After they dried, they really do a great job reinforcing the wingtip without adding much weight.
