Came across these protective sleeves for Macbook/Macbook Pro's in the 99p store, various colours to 13", 15", 17" sizes.
Typical as had one I paid nearly £20 for on Amazon arrive today. This isn't such good quality but does exactly the same job, only issue is leaves a few mm exposed near the flap. Maybe because 13" rather than 13.3" that the Macbook is, however saw similar product in Poundworld which was stated as the actual 15.4" (although not the other sizes) but obviously a penny more expensive!!