Uber,
Rabenu Avrohom Ben HaRambam in his pirush on the Torah, (Parshas Trumah) clearly says that it was like his father had drawn – NOT round…see below link (Ois 32)
http://www.hebrewbooks.org/pdfpager.aspx?req=40223&st=&pgnum=399
Also, read the following link:
It covers the topic well. However, read it having in mind that the author is Lubavitch, and thus trying to prove the Rebbe correct.