Hmmm, so you need a longer focal length to keep further from the
subject. That is odd because extreme closeups are usually done with
short focal length lenses, like the Multiphot above uses 19mm at 1:40
and 120mm at 1:1.

But since the subjects are submerged, longer working distance is necessary.

To use longer focal lengths would require a very long tall bellows.

For example...? Can you give me an idea of what lens (backward?) and what
bellow length? Just a ball park idea...
--
Dr. Leonard. H. McCoy
"I'm a *doctor*, Jim, not a *magician*!"