Endplates and anti-air-bubbles-getting-in-the-way plates are worthwhile low-budget things to try. I'm for them. I could write more authoritatively if I could ever limit myself to making only one change at a time.
But even with allowances for the human tendency to love the results of a project just because it's finished and irreversible...
mehitabel's final rudder, with both a sacrificial bottom endplate and a top anti-a-b-g-i-t-w plate, works so much better than the previous (okay, slightly smaller & overbalanced) prototype rudder, that I'm convinced.
The said prototype proved the worth of moving a rudder aft, and (to admit another inevitable concomitant change) of a spade planform over a skeg-obstructed one, assuming steering is the main thing, not whale impact or grounding.
(I used 'concomitant' for effect. If I'm less than rigorous in practice, I can at least enjoy sounding technical.)
Any endplate is enough better than none, that how much is like peanut butter in a sandwich.
Cheers, Kurt