It looks like ROb at Welsh Harp has a second-hand combi for sale that he thinks might work for a 12 with a bit of work on the cradle. It would obivously not be as good as the ones he makes!
http://www.apolloduck.co.uk/display.phtml?aid=144684 (http://www.apolloduck.co.uk/display.phtml?aid=144684)
Antony