Taking good measurements and checking your local salvage yard is about the best bet.
If you really can't find anything, these guys can make one, but it's expensive. The one for my 47 sedan was priced at $800! I had the old one cleaned out at the local radiator shop.

http://www.rockvalleyantiqueautoparts.com/products.htm

I don't know how much carries over, but there is an active Power wagon site here.
http://www.dodgepowerwagon.com/main/parts.htm