Bron, I have one a friend of mine made made many years ago from a table tennis clamp and a bit of curved metal. I told him to patent the idea but it was too much trouble for him. Since then a company has patented his idea and produced what they call a "Bar Prop" I think. Geoff The Duck uses one. I am sure they can be got at Music Room in Cleckheaton if so Eric the Viking will probably get you one and send it to you.
Cheers Bill