Stewart,
If it is any use to you, when I made the new clapper box, I drew it up in QCAD. A jpeg export is shown below. There might be some degradation in the quality. If you would like a copy, PM me with your email address and I'll send you a copy.

Just one thing to note if you make it to fit the existing carrier. The width of the is shown as 25mm. Instead use 1" square BMS for a better fit.
Regards,
John