Here is a site that shows just what you want.

http://www.enter.net/~ultradad/bowljig.html

Stuart Johnson
Red Oak, Texas
In article WBzUc.178102$eM2.148793@attbi_s51, william kossack
wrote:
I am working on a bowl a bit larger than I've finished recently.

I normally reverse a bowl onto my cole jaws and turn the bottom removing
the tenon or anything else as I put the last finishing touches on the
underside.

This bowl is just slightly too big for the cole jaws I have but I have
others that are roughed out and drying that are coming down up soon.

Now what do I do? I figure that I maybe could cut some plywood to make
a jig to hold the bowl in place but I need a starting point of a design.