I've seen people use a small piece of plywood and a hi hat to go from a Mitchell mount tripod, to a bowl fluid head. But I don't think that type of kludge will work going from a bowl, to another smaller bowl.
Might be cheaper just to rent a matching set of sticks if its a short gig.
Thanks, Scott. But the problem I have would need a REALLY good kludge as I'm using big Sachtler sticks with a 150mm bowl to support a Jib arm with a 100mm base. The weight of the jib pulls itself out of level too easily. When I use the bottom nut from the 150mm head it holds for a few hours so for now maybe that's the best I can do.
I also hate renting because of the hassle of picking up and dropping off or the expense of having stuff shipped to many of the smaller markets where most of my work is. And frankly I'd rather spring for a 100mm heavy duty tripod than spend $235 on a ring of metal. At least that way I'd feel like less of a sucker.