If the hard maple is straight grained it doesn't need a backing unless you are trying to glue in some reflex in which case you could use a strip of maple to back the maple with. For no reflex, but for extra protection you could use rawhide or linen. I have one ash backed elm bow that was gifted to me which has held up well, but never used ash as a backer myself. I have made a number of bows with hickory, white oak, and bamboo as backers with good success.