I've used poly for other things, and it would get pretty heavy. Depending on your application, VERY light fiberglass cloth works well, like .25 ounce. Mix in some of the microfiller balloons stuff in with the epoxy, and it will be even lighter. The microfiller stuff is like an extremely fine powder that when you mix it in with the epoxy, takes some of the epoxys place so it is lighter than the same amount of regular epoxy. It works well when fiberglassing something to paint, but it does make the epoxy weaker so it isn't good for cowlings and firewalls and such.