Peter,
I am not sure how different the C9.25 options are from the C8, however my setup for piggy backing an 80 refractor is with guide rings onto a purpose designed Losmandy mount for the C8.
I use this:
http://www.bintel.com.au/Accessories...oductview.aspx
You might be able to use this:
http://www.bintel.com.au/Accessories...oductview.aspx
Cheers
Sam