I dont think it should even bother us that the trunk is not sealed. We live quite happily with stern hung rudders (for convenience only) Spade rudders are better, but not "enough better".
We cannot justify plowing the 20 foot hull (sin3x20' = 12 1/2"? = increase of frontal area of 200%) thru the water (the sides of the hull are not sealed (submerged) which is why we have all that wave drag) to save a gurgling board.
A board could be made to flex for increasing attack once it sees a loading, However, I dont know how to stop it fluttering.