a lam resin with a opaque white tint. make sure to use acetone or sand the plug before installing, they have some kind of coating on it, that could make the resin not stick too well.
i cut a circle of fiberglass and lay it in the hole, and then cut up some fiberglass to mix in the resin
Be careful... most fillers that are "light weight and easy to sand" reduce strength. Milled fibers and chopped strand increase strength, but aren't lightweight or easy to sand. Same with cabosil. A lot of manufacturers use a combination of cabosil and microspheres to get the better bond strength cabosil gives, but the reduced weight and sanding effort of the microspheres. A dab of pigment makes it opaque.
In my opinion, leash plugs need maximum strength properties. Weight is not an issue with such a small quantity. Go for strength. In big ding repairs needing a lot of filler, I use microspheres, where weight is an issue.