But Olorin's suggestion to use 'U' bolts might be a way to go if I can't find them, didn't think of that - kinda obvious really.
The only ones I know of would be in aluminum.
Plastic would be EZ to make. Just get some square 2-1/2" X 1' thick Delron® or black acetate. Drill a 1-1/2" hole in the middle, drill a couple 1/4" cross holes and split it down the middle with a saw. It would be like the ones in your first link. Similar to the clamps I did for the wind vane except they are a horseshoe shape.
Half the stuff on my boat is custom made.
__________________ Faithful are the Wounds of a Friend, but the Kisses of the Enemy are Deceitful! ........
The measure of a man is how he navigates to a proper shore in the mist of a storm!