After taking a little time off, I finally came up with a semi-decent design for R2's ankle. It took a little shaving of the ankle to get a better angle which was ok since the ankle was a solid resin filled ankle.
Which leads me to an observation about R2's center ankle. Using the ankles that are the club norm is a pain in the neck. The design of these things is poor in my opinion. But they accommodate 3-2-3 positioning which means R2 going from 3 legged mode to 2 legged mode where the legs pivot and the center leg retracts into the droid. But the fact of the matter is very few droids are built for 3-2-3 nowadays so why not design an ankle that has more material that can extend into the foot shell to attach your casters/omniball wheels. That little piece of ankle with one hole in it just doesn't cut it.
A few months ago, I saw one of the R2 members blog with a prototype they built of an intelligent center ankle design. It was made out of wood and I wish I bookmarked it. In my opinion, it made so much more sense than the so called "normal" R2 ankle.
So my R2 center ankle/foot/caster assembly is just about finished. It's pretty level now and I put an extra hole in the ankle to put a second bolt through to lock the ankle down. I'm not 100% sure about whether this will take all the stress of the weight so I ordered a JAG aluminum ankle just in case. When I get that, I may create my own center foot/wheel mechanism similar to how I created my rear drive train mounts. Using a heavy piece of aluminum that extends into and through the ankle and into the foot.
The way my center caster attachment is designed, I can now fine tune and adjust the angle of the caster inside the foot. I think this will work with a single caster but I'm still weighing my options for 2 casters or 1 caster and an omniball. It will probably be modified a hundred times before I'm satisfied.
Sorry there's no pictures but when I test it out and it works, I'll post some.