For the Mark II , I'm guessing you want the fishpaper coupler due to the midbody seperation.

Semroc used to have those, surprised someone doesn't have an extra kicking around.

I think, if you had two of the BT20 couplers, maybe peel one layer of paper off the second one, glue on the first to make it larger od - hopefully to fit the BT30 ?


Good luck with the project, Mark II was my first ever rocket, and one of the early ones I cloned as well.