Peer pressure is a terrible thing. In the end, I succumbed to the influence of the assembled CPR modellers and went ahead with a ball float to indicate the water level. This, they pointed out, would be easier to maintain through an Ontario winter, and Dan MacDougall even came forward with CPR drawings to sweeten the deal.
After a brief search for an appropriate bead, I turned the ball itself from 1/4″ brass, which scales close to the 22″ diameter used on the CPR. I soldered a piece of square brass tube into the hole through the middle of the ball to make the hole square. The hole needs to be square to keep the ball from rotating on the square mast, which is made from 1.5mm square brass.
Maybe I could have pre-drilled holes for the two wires that push the ball up the mast, but I didn’t, and instead I bent a single piece of .08 phosphor bronze wire so that it wraps half-way around the ball and extends down through the holes in the roof. I soldered the wire to the ball and filed much of it away to return the ball to near-round. The float might benefit from slightly larger holes in the roof, and I am quite concerned about friction introduced by painting the mast.
Pembroke:87 
I wasn’t sure how they worked so I printed the balls. Less weight, correct shape, predrilled holes. Painted the post with warmed over testors enamel. Not sure what final tank orientation you will use but after a lot of screwing around I made a special support that looks like a highway barrier in profile with two points of contact on top and printed the tanks upside down. Might not work with the banding overhang you have.
AH
Yeah, I printed the tank at 45 degrees, which enabled me to support all the band ends. This copy had too many supports. The final version kept most of the heavy supports on the bottom of the tank. Only supports on the front are there to enable the bands to print before the rest of the tank on that side. I did consider printing the ball, but the weight should help it to work; indeed, I think it is going to be attached to the plunger (somehow!)