Above are two different styles of water treatment plants found on the Great Northern railway. The prototype on the right is the original 1912 plan, with a very low slope to the roof, and the other is from a specific plant in Des Lac, ND . What is likely to go into the client's layout is something midway between the two -- the 1912 floor plan with a more peaked roof, which is what most of the new plants had by the late 1920s.
I've learned that I can use really cheap supplies for these prototypes. I'm lucky to have an art supply thrift store nearby where I can buy off-cuts of foamcore for pennies on the dollar. I spent less than $2 building that water tower and the two treatment plants.
Here's a cardstock prototype I made to check some proportions. This one is so small, I didn't need to use foamcore. I drew the original on some HO scale graph paper, ran off a copy on my home printer, and built it very quickly.