Based on your discussions, it sounded like the construction of a 31's rudder would be similar a 34's which is why I thought to share the info. I concluded that Karma's rudder didn't have signficant rusting of the metal and that the rudder was not waterlogged so elected to fill the tubes with the foam then to patch and fill the rudder.
Short Pump, VA
1981 O'Day 34
"No matter where you go, there you are."