cool project and please don't think I'm being critical, but I think the grade may have more to do with the traction loss than the material. Any chance you can draw out the slopes?Now on to something constructive- roofing shingles, although very rough only provide traction if the item against it is softer, Shoes, hands (for you roofers out there) kind of like skate-board tape so.....we need something softer than plastic or metal tracks.....I'd grab a can of spray on bed-liner or any rubber sealant. You may even be able to get a good color match and try that. Regardless of how you tackle it, please post the results.H