My Enerjet ('72/final version/Orion type nose) Aero Dart and Enerjet Athena clones are about ready for paint.
The motor mount for the Aero-Dart consists of a 2" balsa bulkhead cut in half and drilled for a 29mm motor tube.
Semroc included some sample (I'm assuming due to the fact that the website does not have them) 29mm to ST-16 centering rings that I used for the mount for my Athena. I'm hoping they make ST-20 and ST-16 to 29mm centering rings a "cataloged" item.