Hi Folks, I've found a clamp that's lighter and smaller than the standard Matthews-type circular Hollywood Grip Head clamp for attaching the fishing-pole "Y" fork and hook holders that many of us use for static boom pole setups.
It's the Bogen Manfrotto 2905 Swivel Umbrella Adapter. It's much sturdier than it needs to be for an umbrella (it can hold a light at the same time) and very capable of holding the shaft of the fork and hook for the audio boom pole. It also has a better adjustable handle for tightening the swivel joint than the Matthew's oval IMHO.

http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=5156&is=REG

That one?

Yep, that's the one. The picture is a little dark but if you look closely at the upper half of the clamp you can see the small hole (and set screw knob to the side) that is the correct size for an umbrella shaft or the shaft of a boom pole holder. This still leaves the upper 5/8" opening available for something else including the adapters for 1/4" and 3/8" that come with it. Plenty of pivot angle too and the main handle is adjustable for position.

I suppose a Lobo clamp would work as well. they're handy for umbrellas as well. It's $24 at B&H. if you don't like lowel then you can buy the iedentical clamp from Westcott for $32 ;-)

Lowel Lobo (http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=31991&is=REG)