A trim wedge for your rudder.  Material is resistant to acetone, paint thiners, fuels and UV resistant.  Proseal would be a good adhesive choice to use for permanent bonding the trim wedge.  For flight testing, a vinyl or alum tape is good enough to determine final size.  The hollow versions are 32% lighter.  Based on what I have observed over the years, .325 - .375 is the typical aft thickness. 
 
Note: the angled version is for RV's with a fwd angled rudder trailing edge.  if you are applying the rudder wedge to a Glastar or similar with a aft angle rudder trailing edge, choose the opposite side (left/right) orientation to have the proper angle orientation.