I believe David Vinzant was first to report how to fit 
the 2x2 round slopes together:

http://www.brickshelf.com/cgi-bin/gallery.cgi?f=15786

And James Mathis posted his CAD design here:

http://www.brickshelf.com/cgi-bin/gallery.cgi?f=18688

I especially like the hatch on his corrugated car and 
have used virtually the same hatch and hinge on both 
types of tankcars shown in this folder.

Nice work and thanks for sharing your ideas.

Casey Mungle