You can always buy a premade polycarb dome here:
http://www.globalplastics.ca/domes.htm
Yeah.. We talked about making/buying one of our freshman a hamster ball to run around Atlanta in when we were looking for covers for our dome and I stumbled upon this site.
We were gonna buy two of these size balls here that is covering the salad bar and attaching them together somehow
http://www.globalplastics.ca/micashop1.jpg
edit: Ok, back to the subject at hand - enough ranting.
As for bending polycarb, there are a few good ways to do it, and a million bad ways.
In 2002 we had a polycarb formed cover on our robot as seen
HERE.
It wasn't a sphere, yet was as smooth as a baby's rear end.
I believe this was achieved by a length or heated up material of some kind that was in the shape of a bow. It was used to form the curves on the robot's cover, and the cover held up all year. (Remember, this was the year that our robot was attacked continually by soccer balls)
I regret not knowing what the actual material that was used to bend the polycarb on this robot's cover was, but maybe I will see it one of these days.
I know it was a very simple device, that was like maybe a heated filament of some sort, and the lexan was bent around the material.