I came up with this when I was rebuilding my 54mm Acme fin can rocket.
1. Parts: 6" coupler that is a good fit for your main tube, 2 Bulk plates that fit inside the coupler, Forged closed end eye bolt with 2 nuts and 2 washers.
2. Tools: Epoxy (30min.), Coarse sandpaper, Knife, saw or dremel with cut-off wheel, masking tape, broom stick or other pusher rod, long dowel, plastic spoon, flash light, drill and bits, wrenches.
3. Cut coupler into two equal pieces, rough up the outside of one of the couplers with the sand paper. Wrap your broomstick with coarse sandpaper at the end make it so you can rough up the inside of the main tube. Rough up the inside of the tube about 6" in front of where your longest motor will end. Tape the plastic spoon (or a metal one you've stolen from the kitchen
to the dowel. This is used for applying epoxy inside the tube. Apply a liberal bead of epoxy just aft of where you have roughed up the inside of the tube. The idea is to get the half of the coupler epoxyed inside of the tube to act as the stop for the shock cord mount. I bought a really long 54mm coupler to use for a pusher but you can make one using a broom stick and a little ingenuity. This is just like putting an engine block in an Estes kit just a bigger tube and farther up inside, it is also a bit like being a snake proctologist ;-). Try to give the coupler a twist as you put it in to spread the epoxy. I would also mark your pusher to be sure you are getting your coupler were you want it. Withdraw your pusher as soon as you get the coupler into place. Keep the tube level in case you have some epoxy that wants to run out. If you are having trouble with excess epoxy make a giant q-tip with a rag and your dowel, use this to swab the excess away (flash light will help here).
4. Take the other half coupler and rough up the inside of it with coarse sandpaper. Drill holes into the center of the two bulkplates to fit the eyebolt. Rough up all sides and edges of the washers and the bulkplates. Degrease the eyebolt threads and nuts with laquer
thinner. Rough up the surfaces of the nuts. Epoxy the bulkplates together and using liberal amounts of epoxy mount the eyebolt into the bulkplates so that you have the componets in this order: nut, washer, bulkplates, washer, nut. Use lots of epoxy to bond the whole assembly together. Afler the bulkplate assembly has dryed mount it into one end of the coupler with lots of epoxy. Leave it about 1/4 from the end so that you can put liberal fillets on both sides. When all has dryed drill four 3/8" holes in the bulk plates so that it will pass the ejection gasses. Be sure to make this assembly really strong as this is the mount. You may even want to turn this into a baffle using another bulkplate with holes in it.
5. Now you have 3" coupler bonded inside your body tube and a 3" coupler with an eyebolt mounted in it. Attatch your shock cord to the eye bolt (tubular nylon or kevlar) I sew mine using kevlar thread, but you can knot it, or combine the two. Now drop the shock cord into the aft end of the rocket and pull it through from the front. This should seat the shock cord mount behind the coupler you bonded into the tube. If the mount is loose in the the tube use some tape to shim it up a bit.
6. This mount leaves the tube clear in front of it, and you can remove it after a flight and inspect the shock cord for burn damage. It also lets you change out the cord. You may want to scale up the hardware and bulk plate attachment depending on the size of the rocket.
Somebody on RMR or RO named this the "Brown Bulkhead"
Leica,
(D.L. Brown)
I hope this comes out clear enough...