As others have suggested in their build blogs, they have made wedges and drill/tapped/bolted the wedge into place.
I went with the suggestion of Mike Senna and Victor Franco made: cut up some pieces from a plastic cutting board. Its thick, has a bit of flex to it and inexpensive. I found a $2.99 cutting board at Lowe's and as ironic as it sounds, cut it up.
Using some scrap paper, I made some simple templates to roughly cut out of the plastic.
Now cutting plastic with a Dremel is...well...a real messy experience. However, its the most effective way to zip it piece out, check for fit, file down a bit, check for fit and repear til you are please with the fit.
They aren't perfect but they do the job. After installing these and driving R2 around, there is a huge difference. After doing the test drive, I wanted to check so see how the shoulder assembly was holding up. I was curious how loose the nylon-insert nuts had become from all the rocking around.
Thus it was take-apart time to check the shoulders. I did find that they were a bit loose and while not dangerously so, I opted to replace the nylon-insert nuts and also added a flat washer for good measure. With the dome plate removed and the dome off, I checked for anything else that might require some snugging up. Everything looked alright, so now the batteries are all topping off their charges for this week's events.