The cases WM sells is nothing more than a fancy cat litter box with a latch on lid. I looked at one some time back and there is no hinges for the machine, no walls to keep the machine from shifting around, just an open tub with a lid. Personally I wouldn't waste my time or money on one.
Check you local sewing machine shops. I just got a case for our 518 for $4.00 yesterday. That is my suggestion.
Joe