A while back on a Wetpixel thread I pondered the feasibility of using plastic soda (or pop as we say up here) bottles to offset the negative buoyancy of my rig: http://wetpixel.com/forums/index.php?act=S...st&p=107218 
Disclaimer: Its not pretty for those of you who use cameras as chic magnets, and I make no claims on the safety aspect of this application.
I have since reduced the concept to practice, added a new twist, and tested it in the field. The new twist is adding schrader tire inflation valves to the bottle caps, and pre-pressurizing the bottle with the BC inflator hose and a tire filler adaptor.
Since the soda bottles have ridges on them, they do not lie flat against the strobe arms. To reduce play between the arm and bottle I applied a zip tie around the duct tape to cinch up slack. This turns out to be very effective, and the combination is rigid enough to adjust strobe position via the bottles. The quality of duct tape matters, some stretch more and rip easier than others (I was employed by 3M, and still own stock). I would occasionally re-cinch the zip ties to take up slack from stretch.
The tire valves are from an auto part store and cost about US$0.39 each. The bottle caps are right off the Pepsi bottle (I do not own stock). A 0.5" (12.7mm) hole drilled into the cap accommodates the self sealing valves just fine. Total cost per arm: US$1.39, including beverage of your choice.
Excessive buoyancy can be compensated by adding water to the bottles.
Results:
With my Aquatica D200 and pair of Inon Z220S strobes, I can achieve neutral buoyancy with wide angle setups (sometimes having to add water to the soda bottles). With the 105VR and Fisheye focus light, the rig can still be handled with fingertips.
I made 38 dives without having to replace the duct tape assembly. I could have easily made 38 more dives with the setup.
The lift created by the dome port can be offset by twisting strobe arms to position the bottles further back. Macro set up with the arms forward compensates for the negative port/lens. The separation of positive and negative centers of buoyancy creates a torque (twisting force) when the rig is not near horizontal. Vertical shots require some muscling. In the future, I want to try two smaller bottles placed closer to the Z220s, and a 20 oz (0.6l) directly under the housing.
Notes about safety:
Soda bottles must withstand at least 60 psi, since that is the pressure of a hot soda in the trunk of a car. There is whole "sport" of using plastic soda bottle to make water rockets, there is a lot of info online wrt to this safety, altho this is a boy scout type activity, so its very conservative. At first I was quite sheepish about how much pressure to apply, using a tire gauge, progressively went up to about 80 psi. Then I attached it to my bicycle pump, and maxed it out at 130 psi. Then I dumped what ever my 2nd stage is set at into it. I then filed it to 100 psi, taped on a broom handle and beat it against the garage floor, finally I whacked it with a a sledge hammer. No failures were encountered. It did fail when I stabbed it with a knife and shot it with a bb gun. The failures were benign, hissing, not exploding. Now I regularly fill them up to 80 psi which will keep them fully inflated to 150 fsw.
