Hi Pete,
I have lots of fins with that mounting system but they are all very small in depth. However, I found one that is 65mm deep that I believe would work (see pictures below).
I see you contacted Airush international, they can't help you with this unfortunately.
Last year, I donated lots of older fins and Mark_Australia from this forum took some.
www.seabreeze.com.au/Members/Profile/Details.aspx?member=Mark%20_australiaMark runs a surfboard repair business in Geraldton. He might be able to help you find a matching fin or help you make one custom.
www.facebook.com/MOzCustoms/If you wish to get that 65mm fin I have, send me a PM

Thanks
Christian