Stop heat loss through unused chimney duct
It sounds like you have a convective loop, in which case the reason why the fiberglass insulation was not as effective as you would have liked is because fiberglass is air-permeable. Since you have access to the duct itself, try sealing it off with a section of fitted and caulked insulation foam board or something. Doesn't matter where in the duct you do it; the important part is to stop the movement of air to break the loop.