When studying Chumash and Rashi there doesn't seem to be any discussion of any pleated excess on the corners as one might see on the corners of a tablecloth as it hangs over a square table.
It would seem to me that when hanging the coverings over the structure of the Mishkan that there should be a pleated excess in the west side corners, and even on the east side with the first covering according to the Baraisa and certainly with the second covering.
If the coverings were secured with ropes, maybe they were pulled outwards and so there was no pleat, however, it seems that the coverings hung straight down as the Posuk tells us that the different coverings covered different amounts of the beams.
There is a depiction of the Mishkan on this web page http://www.modia.org/tora/chemote/teruma.html that is taken from an authentic Torah book - The Tabernacle by Moshe Levine. There does seem to be pleats at the back and they are tucked in.
Although not an authentic Jewish source at all, this webpage shows how the corners may have looked like http://thedeserttabernacle.blogspot.co.uk/2010/05/exodus-3518-staking-of-tent-of-mishkan.html
Any help in enabling a clear authentic Torah view of this would be very grateful.