I’ve recently finished installing a fan for our lanai area.  One of the biggest hassles was finding a surface mount box compatible with the size of the fan switch control.  Regardless of type switch, they all have a similar design requiring about 18 cu in of junction box.  Most damp area, surface mount boxes are a about 14 cu in.  I would up using a Legrand Wiremold 2300 series plastic surface mount box  and paired that with a Hubbell Bell weather proof in-use cover to provide a damp-proof housing for the fan switch.

Installed Surface Mount Box (with window!)