I throw out some ideas for you to consider. I have bought full length coupler tubes from Hawk Mountain for their standard size tubes. So if you want extra length couplers you can make them any size you need. I call them full length but I remember a 3' length of 75 and 98 mm coupler tube I bought. I use them for my payload bays or electronic bays.

Next, here is a suggestion it you want better fitting coupler tubes. Many times the coupler tubes are just too loose in the body tube and it requires several layers of masking tape to get the snug fit. I have vacuum bagged layers of carbon tissue to build up a slight thickness on coupler tubes to make them fit better.

Doug

Thanks Doug, these couplers need to hold up to mach 3 flights. Also I made the fiberglass tubes so they are not a standard size that like hawk mountain would even fit. I mean I have an idea of how I would build them, but I am real interested in how to determine the real structural strength needed as well as anyone who has made them and how they worked.

Mike, I've made a few and go 1.33 times the number of layers of the body tube. No scientific reasons to back the above, but so far I have not folded a coupler. The "To 100k" group folded a Hawk mountain coupler in the last attempt. The sustainer was made from 3" "fannin' tubing. and it folded right when the O5100 burned out. I did the above in my 5" rocket but then chickened out and added two more layers to the inside just to be safe. As far as length goes...I've just been eyeballing what looks 'right'. Sorry for the not so technical reply.
Tony

Thanks Tony, sometimes experience is the best teacher.