In reply to idiotproof (Buxton MC):
1) Needs to be made of Silicone/TPU so it doesn't crack after repeated use and can be used at lower temperatures without going stiff.
2) Opening needs to be reinforced and have a roll over velcro type closure
3)Transparent both sides (some aren't!)
4) Take a large map area and have the ability to fold in half and secured with velcro
5) have a strap so you can hang it round you neck and look a complete clueless wombler and I can spot you a mile off (well perhaps not this one!)
Good luck, as I can't seam to find any now with all those features! The material is the most important!