For anyone who wants a cheap, decent length footmuff that fits a BJCM I just bought one of these in the sale
www.mothercare.com/Mothercare-Deluxe-Quilted-Cosytoe-Black/dp/B003UU3CEI?ie=UTF8&ref=sr_1_9&nodeId=44338031&sr=1-9&qid=1333129969
No slit in the back but it still folds if you put your hand under it