I have some experience with using the various ells that are available for ABS pipes to make curves. You can see the results in the ajustbable rod holder shown in the photos below.
Pieces.jpg
Outboard-close.jpg
If I were building something like this, I would position a 90 degree ell close to the scotty rod holder and build off of that. This would start the curve process and reduce the center of gravity. From there you could use some of the other angle ells to position the pontoons where you want them to be.
Heating the PVC to make it curve is another option but in my experience getting the pipe hot enough to work with, is a bit of an art. It is really easy to overheat the pipe and cause it to start to bubble. I also suspect that PVC which has been heated and cooled a couple of times becomes more brittle than unheated material.
Once you get the design down, consider mounting one pontoon on the stern and the other at the bow. It might look lame but I bet you would get maximum stability out of that setup.
Bob