Squareback will fit in a blade headcover

It will fit but won't be ideal and will stretch it out. You really should go with a mid-mallet, IMO

Just spoke with folks at Scotty Cameron customer service and they recommend the mid mallet headcover for the square back putters. They indicated that the blade headcover would be a bit tight. I had a couple of the Notchback putters and used the blade headcovers and they worked fine.
the Notchback are a bit smaller than the square back. I do not know how much the square backs changed since 2014. The grey headcovers they have on line are nice and retail for $49. If you prefer to place your order via telephone or have questions regarding your Studio Store order, you can call Customer Service Center toll free at (800) 214-0213. Their representatives are available Monday through Friday from 8:00 AM to 5:00 PM PST, excluding the holidays listed below. You can check to see if they have any period correct headcovers available, otherwise it will be eBay for $75.

Mine fits in the blade cover, a bit tight but has stretched after a few weeks and fits nice now