I actually made some the other day from a waterproof jacket and a woolen fleese jacket both purchased from the womans section of the help the aged shop. Total cost, 8quid, 5 hours with a needle whilst watching TV and I have some extremly functional hand muffs.
Ill post some pics so folk can see but I found that if you cut vertically up the back down the middle then there is enough fabric to go round the bar guards and back with a little to spare. Just right!!!