Richard they come up on ebay and regular dealers will get you one. but a stronger one can be made if you have any old clutch plates lying around two - one steel one friction - drill and bolt them together with perhaps three or for nuts and bolts, also fix on a good length of steel strip (remember to leave room for a socket in the centre).
they need to be lined up centrally as you would expect and a good length of steel strip allows it to come around and anchor on the footrest or the ground.
Here's one I made earlier
