So I bought a set of jumbo and a set of large Stix floats. The jumbo should get me to near neutral, and then the large to trim and offset any weights I add to the dome.
If I get the chance, I will try to put the rig in a wheelie bin full of water so see if it is at least neutral.
As an engineer at heart, my thoughts turned to trying to find the center of balance and buoyancy of each component - dome, housing, strobes - so that each part is perfect and changing strobe position or port will always end in a neutral, well balanced system. But not right now!
To answer a few of the comments above (thanks for these) I only shoot wide-angle at the moment so will move the arms back to offset some of the dome buoyancy.
I use wired strobes, simply because the housing came with wired bulkheads and a Heinrichs & Weikamp TTL converter.
Hopefully, I will report back after my trip. Thanks for the help.