uhmw plastic jig stock - 3/4'' thick
if you screw the runner in from the bottom then it is possible to spread the material enough to make up for a small gap (on the order of a few thousandths of an inch). by the way there is a glue that attaches uhmw well to wood: 3m high performance industrial plastic adhesive 4693h. it is strong enough to make screw fasteners redundant.