had some trouble. I made the interstage too small.
rather than correct it, I tried to taper the 1st stage to fit better hoping the taper wouldn't be noticeable, but I'm disapointed with the results. To repair the damage, I am putting multiple laminations of cardstock on the interstage to build up the diameter. When it gets to an acceptable diameter to match the old diameter of the first stage, I will then put the skin of the interstage on. This seems to be working well. for the tapered 1st stage, I will put a 1/2" thick lip on the top of the first stage to undo the taper, again using multiple laminations to get to the right diameter. Once it is the right diameter the final stringer skin will be put on, and thus returning the 1st stage to a cylinder. as the inside top of the 1st stage was also damaged with my ill thought out 1st attempt, a new grey skin will also be applied to it. The lamination technique seems to be working out quite well to make this over-sized model.
The interstage also requires what I am calling a snap ring to attach it to the 1st stage. it's just a piece of waxed cardboard (I think a cereal box will do) made into a ring to fit inside the interstage and bridge the 1st stage, with the wax side out. About 1/4 of the ring can be folded in to allow you to fit the ring into both the interstage and the 1st stage. Once this is done, reach into the interstage and "snap" the 1/4 folded in section back to a ring, and it will hold the interstage and the 1st stage together quite nicely assuming you built the ring to fit as tight as possible.