Step by step...here we go.
The pdf is my drawing redrawn and scaled to fit.
The shoulder fits a BT-55 with a 1.59" lip for the upper second stage shroud to fit onto.
The first pic is the part ready to turn.
The 1/2" wood dowel goes completely through the lower section and into the upper.
The gap with the dowel exposed (I'll deal with that later) will be under the interstage strut.
The second pic is the rough shaped part.