Hi Deisel,
Not sure how far you got with this, I'm making a small tumbler at the mo, but whilst looking around I found these two vids on YTube:
http://youtu.be/oh2kdAhk60I
http://youtu.be/opXT3CUaox4
Both seem well designed / built and I can't see why they wouldn't scale up to a bigger barrel...
Oh, and m_c - try this:
http://www.abrasiveengineering.com/bkmassfin.html
...which I found the link for here:
http://www.candmtopline.com/faq.html
w./