I had a Nexus D90 housing and did the following for my housing focus light needs.
There is a black plastic block on the front, beside the port connection. The block is threaded with an usual tapered thread and Nexus sells (or used to sell) a very expensive adapter for this connection. The block screws into the housing exterior and there is no penetration inside.
I took the block to a machine shop and had them make a duplicate in aluminum. Instead of the tapered thread, I had them install an 8mm thread. I then bought a Ultralight ball connection for that thread. I painted the aluminum block with galvizing paint.
This worked great for all the time I used the housing.
I would show you photos of it but I no longer own that housing.
I looked online at a picture of the D300s housing and it has the same block so I am guessing yours does too.
This was a really nice place for the focus light as it was quite close to the lens and easy to aim/use.
Hope that helps. let me know if this is not clearly explained.
Several years ago I learned something very, very valuable (from Alex Mustard, in fact): the best images are carefully planned, and the dive is constructed around that plan.
The lens, the port, the lighting, the time of day, the position of the subject - they all matter.
"General purpose" kit yields snapshots: zoom lenses, multi-purpose domes and infinitely flexible strobe arms may let us photograph a multiplicity of subjects, but how many would we photograph well?
Carrying two set-ups just increases the tendency to jump from subject to subject, without giving each one the consideration it deserves, and without committing the dive to the aim of that one, perfect, image. My best photographs have been taken when I've carried a very specific camera set-up, and worked creatively to match its characteristics to the potential of the subject.
I think that flexibility is a blind alley!
Your comments, in a nutshell, describes several reasons why most of us take glorified snapshots and Dr. Mustard (and others) generate art. Taking a standard dive boat outing rarely allows this level of planning and control. Knowing what to do with the availability of such planning and control would seem to be another.
One might think they could go to the Apeks website and see the weights of each device, wing, backplate, etc. Then you could figure how light a system you could make. They list considerable information but no weights for the wings...just the lift weights. sigh...