Two Tracks is correct: you can't glue to UHMW polyethylene. It's like teflon in that respect. Also like teflon, polyethylene plastic creeps (i.e. constantly deforms under load) badly, so if you made a riser out of it and kept the bow strung the riser would deform over a number of hours - especially on a hot day. Oh yes, and it's not very strong or stiff compared to wood.