That little piece of BT-55 forms the middle of a barrel-shaped transition between the main body tube and the “nose cone”. I glued in BT-50 to -55 centering rings. Then I cut out the upper and lower transition pieces, two of each printed on cardstock, with gluing teeth on one piece of each pair. Here they’re glued together. Then I put the lower transition piece on, glued to the inner BT-50 and the main body tube; I glued the center section to the BT-50 and the lower transition; and I glued the upper transition to the center section and the inner BT-50. And I put a chunk of wood on top. That’s not the “nose cone”. It’s just to hold the glue joints together. And here’s how it looks so far. That is the “nose cone”, not attached yet, nor will it be until I’ve finished the rest.