Graham. I think I have two of these in my garage. Made by a mate of mine in the US and can therefore hold the largest soda cup the Americans can come up with.
Designed to slip under the edge of the centre console on a C5 and rest on the floor in the footwell. Fold flat when not in use.
May work on a C3. Yours if you want them. Send me a PM and I'll see if I can find them and bring them to the Nationals.