I got one for a fiver in Milletts in Plymouth. Plenty online but make sure they are big enough to prevent you turning an OS map into an Origami course They come with a long cord that goes around the bar clamps.
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Caution - the poster may well have been drinking........
There are two eyelets at the top corners of the map case. The string supplied is long enough to go around both ends of the handlebars and above and below the map case to hold it in place. Tie the string with a bow, because if you use a granny knot you will have to cut it off later. You can lift the map case up in order to refuel.
Cheers Pete,I just got one from Millets and will attach it tomorrow,it would be easier to keep it in my pocket so I think I may put a new jacket on Xmas list
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Another customer finds himself £20 better off at Teign Tyres and www.teignecotuning.co.uk