As long as their shells don't touch the support bar if you were to use the old floating dock, you can just put something across the bar. Like, a piece of plexi, or any plastic, or even a piece of wood.
I thought I was going to have a problem with that for Frankie, so I put a scrap piece of plexi
(it's got holes in it because I was going to use it as a basking ramp at one point. It's tied to the atba wall on one side, and just rests on the middle support bracket.
I've never seen her even try to get out that way. Then again, she's pretty lazy
Same sort of idea for Ralph's except I built the extra wall right into the atba.
It works too... The other day Ralph was trying his hardest to lift himself up there! He had both paws on the middle support bracket and was reaching up with his neck! O.O