A neighbor has removed a ZC fireplace and is doing a DIY, alcove install in its place. The chase cap has a 13" opening hole with a 2" high rim that they'd like to keep. It's a big cover and still in good shape. What would you recommend for centering the new 6" chimney pipe in a watertight fashion at the top of the rim? I was thinking of a flat metal donut, 8" ID and 12" OD. They will need to get this fabricated. Are there off the shelf solutions or other ideas to do this correctly?