So a few years back we used something like this:
They use globe motors and have a high reduction. They were originally used for satellite dish positioning. They are pretty heavy, but we took one apart and were able to take the weight down. I apologize I don't have any exact figures for that.
They do provide really nice mounting options, and the decent sized through hole offers some unique possibilities
I found them here,
http://www.skycraftsurplus.com/index...OD&ProdID=1016
but it looks like they have stopped selling them. There was another place online selling them, but I can't seem to find it. These have been surplus items for years, so maybe people are finally running out of them. Stuff like this exists though that may help with making a more robust turret.