Thanks Marc , how big a fan for a 10” mirror? What about vibration?
I have fans on my C11. Get a couple of the smallest plastic sleeve fans you can buy. Vibration is minimal. You don't need to run them all the time either. Just to keep the mirror close to ambient.
As Marc said, Jaycar sell a range of small fans. I got 3 maglev fans for my 20" dob, and they also sell the switching components (separately) you need to get it wired up if you feel so inclined. It's a bit of a challenge if you're an electronics gumby like me though. The fans are very quiet with minimal vibration - you'll barely know they're on if you get a decent quality one.
It would be nice to take a battery to Jaycar test out vibration levels every fan is different brushless or ball bearing. Some are excellent and some are a little rough. If you have the time it wouldn't hurt being selective.
Maglev fans are good for vibration and longevity. That said, you're never going to lose a fan to old age - more so vermin, condensation or corrosion. Look for the little rubber push-through fixers, rather than screws. These are typically bundled with PC case fans for silent applications. PCCasegear has all that, though Jaycar may too.