Exterior thermal windscreen cover

We are after some much needed feedback. We want to buy an exterior windscreen cover for our Fiat Ducato (we already have an interior one), we have looked and looked and now need to make a decision. We have seen cheaper one's £50ish and dearer one's.....does anybody have any advice or experience of one's to avoid or one's to buy before we bite the bullet and buy? Your thoughts will be gratefully received.