Thanks everyone for taking the time to respond.
Hans - I have googled variously for images of ball grips, bottom grips, hard rubber balls, etc., with little luck so far (although with some fascinating results).
John - thanks, I thought of a pistol grip, but want something to allow the weight of the lens to rest in the palm of my hand while I focus manually.
Hugh - the screw in legs could work - I will look into that idea first.
Akira - many thanks for your research. I have an idea that the grip that I have was a Manfrotto item, but apparently no longer available.
I will also look at gluing a 1/4" threaded shank into a golf ball. The golf ball will be easy enough to get, but I will need to rummage around Yodobashi Camera or a Tokyu Hand hardware store for the shank.
Mike.