Hi.
There are carbon strobe arms available for a long time. They are from a french company and you can buy it from internet (http://www.extrem-vision.com/Elements-de-bras).
I myself have and use 8 of them (2 of 20 cms, 2 of 16.5 cms and 4 of 12.5 cm).
I use ULCS clamps with it, and works fine.
As they are the producers, if you ask them, they can make the arm the size you want.
I had 2 of 30 cms but I found that they were to big, and replaced them. Those days I had 2 ikelite DS125 and I found that as the arms are quite light and long and the strobes were bulk and weavy, it was quite dificult to use the rig underwater, speacially when there were strong current, a lot of vibrations were transmited to the housing.
With the shorter arms. I have no problems. Quite happy with them!
Regards
Pedro Alves
PS - now I have 2 Inon Z240 and a Canon 40D in an Aquatica housing.