EZ Spreader, an Amish farm supply company, makes a fiberglass bed wagon with superior weather handling.  The entire 20"x40" bed bottom is one formed piece of fiberglass, with no wood in the bed. Red painted hardwood racks then slide into the holders, or plastic red racks are also available.  The undercarriage is also in red. 

Features we love about fiberglass wagons:

• Solid one-piece construction in the bed
• Largely weather-proof, especially when ordered with the plastic racks
• Durable white sheen hard to match with regular paint or plastic
• Unique--Fibrous-glass reinforced epoxy plastic as a wagon bed is truly unusual, but works exceptionally well!