I checked this morning. The 18mm threads are compatible with the Estes staging couplers!
13mm to 29mm
View attachment 307371
I think this is the part from Thingiverse, simply scaled as noted above.
I think the niftiest part is the thaw 18mm size works with the staging adapters. The 29mm size isn't compatible with the Estes - the tread's a different pitch.
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