So here's what I've got to work with so far:
the Butterick pattern
a web image of the actual robe:
and this awesome costume diary of the drafting and construction of a Snape costume. Snape is shown in robes very similar to academic regalia and the author's pictures of the drafting process will, I think be very helpful.
Lastly, I actually went to the bookstore during the time when regalia rental people were there. They let me take lots of pictures of the doctoral gowns. I will post those as soon as I get the pictures formatted properly.