Oups ! D'oh ! Hummmm !!

Say that I am building a mid power rocket called Kraken that should fly something like E or F motor. And say that I forgot to fix the shock cord to the motor mount who is now glued.... oups. I am sure it must have happened to someone here. Is the Estes technique good for that kind of motor ? Thank you

That's easy to do. You could have a couple of options. If it were me, I'd try and drill a hole through both centering rings, and pass the shock cord down through them. It may help to insert a tube through the centering rings to help guide the shock cord. The cardboard tube that the igniters are packaged in with Aerotech reloads is a good size and length. Then, loop the shock cord around your engine mount, lasso style. You could tack in place with glue, or not, and have a replaceable shock cord. The advantage of that is the shock cord is wrapped around the densest and hardest part of your rocket.

Marten's suggestion for a removable shock cord approach is a very good one.

But to answer your question, yes the tri-fold shock cord mount can and will work. I'd use light cardstock if you had it and tie a couple of knots in the (currently) free end of the Kevlar just to give it a little more to hang on to. But I have been using tri-folds with Kevlar for some time now, even on models that size/weight, with no issues.

If you do Martin's method, you can superglue the end of the Kevlar to a skewer, and then push it through the hole you made. I'd try pushing it up from the bottom, instead of the other way around... Once it's in place, snip the skewer off, tie your knots, and you're good to go. Nice thing about his method, you can pull the shock cord down and inspect it for damage after flights. If it is damaged, glue the new one to the old, pull it up through the holes, and Bob's your uncle.

The above-mentioned methods are probably preferable, but if they prove difficult then another method would be to epoxy the end section of shock cord inside a suitable length of coupler tube and then epoxy the coupler into the body tube just above the forward motor mount centring ring. This is a technique that I have used in the past with good results.