I use "Quilting Template" plastic I purchase at JoAnn Fabrics. Easy to bend, easy to cut - but plastic so it will not rot.
Personally, I would not bother to try capping the SAS w/ black gravel. Two reasons. First - although lightweight, once sufficiently waterlogged it will not really float. Secondly, because it is lightweight the heavier gravel will eventually work its way to the lower level underneath the lighter SAS.