Thanks for the sketches!
FYI I finished the dock on the ice. Pics below.
Right, wrong, or otherwise, I've been cleaning the fish on the dock and tossing the scrap in for feed/fertilizer. As for critters, I have a pr of Mallards that likes to come and crap all over the dock already.
I was kind of thinking for the cleaning board I'd use a flange screwed to the underside and set the pipe length to drop into a dock pipe. It wouldn't be adjustable height, but simple.
For the feeder I'm thinking a simple sleeve of pipe to slide over. The posts are on the outside so it will slide past to the bracket. Good thought on the thrust. I'll probably just use a short collar of the same tubing so it is steel on steel instead of aluminum. Hoping to avoid any bracing since the bracket should be pretty short to center. I'd keep it close enough to just clear the upright when rotated.
Essentially I---I
On top of the outer tube would be a square plate that the feeder (TH125) would sit on. The plate would swivel and lock in place as would the main upright.