Ian, couple ideas.
First, I thought I'd get some sort of shelf that would mount on a second tripod. Then, I saw this little laptop table that folds up small enough to fit in a computer bag. (?)
Finally, I purchased an older model of this:
http://www.bhphotovideo.com/bnh/cont...ughType=search which I found in the dvinfo classifieds.
Haven't used it yet, but it looks real good.
Other ideas:
http://www.bhphotovideo.com/bnh/cont...ughType=search Actually, this is the platform that's on my new (old) superclamp and ball double ball joint. Looks like this could mount directly to a tripod.
Try a search at bhphotovideo.com for bogen platform - that will turn up a lot of the various bits and pieces.
http://www.skymall.com/webapp/skysto...19&catId=14200 (but I'd never order direct from skymall again, was very dissapointed on another purchase a few years ago. I'd try to find the item from another supplier).