If I were doing it and going with Losmandy as I am prone to do, I'd try part number
VSCT17. Here is their vixen compatible products page:
http://www.losmandy.com/v-series.html
According to the Losmandy web site, that part fits the newer C9.25's, however OPT's web page says it fits only the earlier model C9.25's, so a quick check with Losmandy or a local vendor would be in order.
List price is $65 US, or around $125AU shipped from overseas. Telescopes-astronomy.com.au lists it as $90 + shipping. I've purchased Losmandy gear from them in the past without issue.
Regards,
Eric