Now this was back in 1977,
Mounted two 4 inch fans.One pushing.One pulling.One fore.One aft.Did it for heat circulation but took care of the moisture under the floorboards to.Hate cold floors.Covered them with expanded metal that was coated with something thick and rubbery.No clue what that was.
Cant remember the brand i had but was something like this.
3" and 4" AC Axial Fans
There are 12 volt around to.Just google 4 inch fans.
It worked for me