PS: These are a couple of designs I'm looking at myself ... thought I'd throw them out there (I haven't tried them yet).
(Notice the wheels)
http://www.poultrymad.co.uk/housing/tspecials.html
http://uk.geocities.com/arksandhutches/page1.html
Hope this helps with your ideas and planning...
One of the advantages I do see about the Forsham ark triangular design is that it strikes me as not tip-able in strong winds. So maybe keep the design, but expand it - or build 2 arks. Also - consider how much time your birds may spend out of their enclosure. And - consider that one breed may...