I think what I would do is get a slab that's not too thick, but sanded flat on one side, then start clamping it to the riser profile (just put some tension on it) with some quick clamps or spring clamps and heat it a bit with a heat gun until it pulls up tight with little to no effort. It's tough stuff and fairly flexible anyway. If you get it too hot, you'll know, because it will stink the place up.