In my experience adding pvc as flotation adds little usable buoyancy, as do the ULCS buoyancy arms.
A much better solution are compressed foam floats, made by 4th Generation Designs and Marked as Stix Floats for Ultralight Arms. I've dialed in all of my systems with these floats, some of which were as much as 4 lbs negative before.
We don't have them on our site, but they have been selling well. There is a picture in the wetpixel dema show report from the Backscatter booth that illustrates the floats.
DEMA 2007: Backscatter :: Wetpixel.com
They are sold in four packs for $35, and the jumbo floats add 1.6 lbs of lift to a camera system. You can fit 2 Jumbo floats on an 8" strobe arm.