Looking for advice on printing a tricky part, a scan of seabed wreck.

[Davo]

Logically - I thought - if I set 100% support from the build plate only. It should have filled the space under the floating shell with solid support.
But it doesn't generate anything.

That works in Slic3r - but not perfectly. I'd like to be able to set 4 or 5 solid top shells.