If you have any type of vent on the boat then just glue a 12 volt computer van in it and hook it to the battery. A solar panel to keep the battery charged will produce more than enough power to also run the fan.
I've got two in the back of my boat in some clam shell vents on the back of the boat. I've unscrewed the deck plates in the anchor locker on the front so there's plenty of fresh air pulled into the boat and the computer fans pull it through and exhaust it out the back.
This has worked AWESOME. We used to have a mold problem but once I installed the fans to exhaust out the back it has made the most incredible difference AND it dry's my rope anchor rode and anything in the lazarette that accidently gets put up wet. Not sure if this will help you but it's a bunch cheaper than a solar vent and from what I've seen the fans last just as long. The best part is that the fans are only about 10 bucks or less each so when they wear out (and mine have been running for 3 years now) you can easily and cheaply replace them.