How would I make a whiteboard for an odd sized space?
Dry erase paint is available from a few different manufacturers. 


IdeaPaint 
Rust-Oleum Dry Erase Paint


Are a couple examples.

When I was in college, I bought a sheet material from Home Depot that had a smooth white finish.  I don't remember what it was, but it worked well as a makeshift Whiteboard.  If I remember correctly, it was a thin sheet of MDF coated on one side with acrylic. They sold it in 4x8' sheets, which could be easily cut to fit. The only down side was that if marks were left on it for a long time, they had to be cleaned off using dry erase cleaner spray. 

It may have been Thrifty White.