Don't know if this is what you're looking for but there are a couple of bt-55-60 transitions on this page. You would have to drill your own hole if making this into a boat tail.
Great find. I will keep my hopes up for a plastic version but if nothing materializes from this post, this balsa tapered balsa adapter is a great option.
K'Tesh posted this a while back in a separate thread. This is the Estes Plastic Tube Adapter (Blowmolded)(BT-60 to BT-55)(or BT-55 to BT-60). These parts are often found attached to the PNC-60NA for payload sections (e.g. Scrambler II, Loadstar II, Warp II, Laser Lance).