The flange sounds like a good idea. I had planned to use a 3/4" hole saw and a jig saw and cut through to the other side of the 2x4 instead of routing out the pattern. Then add a 1/8" ply backing to the 2x4. Adding a circular flange onto the ends of the crosspiece should be easy enough. Two backing sheets of ply could then be used, with the center of the middle one cut out so that it would act as a spacer for the thickness of the flange.Might need to keep the whole thing waxed up well for it to slide easily.

I can see how this could be adapted for use with a rudder as well.

: The crosspiece can still get out if it gets at an angle, so the perfect
: design incorporates a "flange" at the ends of the crosspiece so
: it cant get out at all.