I just returned last week from Lembeh and Raja Ampat (photos at Lembeh and R4 photos), where I tried several new toys with my Seacam housing.

1. James' buoyancy collar(s):
Rather than making one big collar as James did, I made several smaller ones (1-2 layers of the 1-inch foam, which could be "stacked" depending on which lenses/ports I used). I held the collars on with a sophisticated rubber-band system , which actually worked surprisingly well. First, another BIG thanks to James for coming up with this idea and sharing his process with us. It made a significant difference in the Seacam's previously VERY negative buoyancy.

Initially, I used four layers of collar (i.e., 4 inches worth) with my macro lenses (Sigma 150mm and/or Canon 100mm). This left the housing overall still slightly negative, but FAR closer to neutral than before. Unfortunately, all that buoyancy on the lens made the housing want to constantly rotate "up" (have the lens face the surface), so I either had to struggle with it (wrist fatigue for the opposite reason I had with no collars) or swim with the housing facing up until I was ready to take a shot (which is what I ended up doing). After awhile, I actually took 1-2 layers of collar off, and found that just having 2-3 inches of collar -- while not providing as much overall buoyancy to the housing -- seemed easier to handle, and still provided a significant advantage over no collars at all.

BTW, the 6-lb foam James recommended showed NO signs of compressing, even after 55 dives, several to 100+ft.

Going forward, I would like to create either some sort of foam "plate" to screw into the bottom of the housing itself, or some sort of foam cylinders to go around the strobe arms, in order to provide buoyancy to the housing, rather than just to the ports. I think this would allow for more balanced buoyancy (in conjunction with still using a few layers of collar on the lens port).

2. Woody's diopter
At someone's suggestion, I purchased one of the biggest (110mm collar as I recall) Woody's "wet" diopter, which can be used (with a little effort) on the 115mm Seacam macro lens port. I found this to be quite functional, as I could take photos of fish, etc. when the diopter wasn't on, and yet quickly (well, fairly quickly) get the diopter on for macro subjects. The only time I noticed any "problem" was using my Sigma 50mm macro lens, where the diopter significantly "cropped" the picture into a smaller circle. But overall, I think the Woody's diopter is a good, inexpensive way for people with Seacam housings to add a wet diopter to their rigs.

3. Canon 500D screw-on diopter for Sigma 150mm lens
I also acquired and tried Canon's 500D diopter with my Sigma 150mm lens. Since this is a screw-on diopter, using it together with the Sigma 150mm lens really commits you to shooting macro only for that dive. While I think this combo did a good job, I ultimately preferred leaving the 500D off and simply having the Woody's diopter available to put on and off during the dive as needed. I will likely only use the 500D when I know I'm going after very small macro subjects on a given dive.

Note that I'm using a full-frame camera; all of the above info relating to diopters will be somewhat different if you're using a camera with a crop factor.

I would suggest against locating a considerable amount of buoyant material below the housing.

From my testing I've learned the importance of locating buoyant material above the ballast. Otherwise, just as you've experienced, the tendency for the rig to go belly up is always prevalent. Encapsulating the strobe arms in buoyant material (core-cell) worked best for me. The strobe arm positioning for most image captures is relatively balanced and above the housing. The form around the strobe arms does not have to be tubular. Several divers now have adopted the rectangle strobe arm form created when gluing four flat sections of 1/2" stock around the existing strobe arm.

Balancing the buoyant material in several locations as you suggest is a good idea for trim in the water. A small amount below the housing, some more around the port, a strong presence around the strobe arms, etc. Fellow diver Evil Bill suggested to distribute the buoyant material directly at the source of the negativity whenever possible.

I think the combination of Mark Furth's Delrin tube research along with what we've learned about buoyant material applications holds exciting potential for the future of diy strobe arm projects.................

Good luck and thanks for sharing your experience!

Good to know...I'm going to try using the same 6lb foam to make some arm floats. I already have two ULCS floatation arms, but I need a LOT more buoyancy on the arms to help offset the overall weight of the Seacam housing.