I've come up with a new method (well, not really new, I'm sure) of shock cord anchoring.
Like, I am sure, most everybody else, I have a number of scrap ends of body tubes sitting around the workshop. I was actually cleaning the place up and throwing some of these scraps out when it occurred to me, "Hey, I could make some use out of these."
So, to make a shock cord anchor, I cut about a 1" long segment of scrap body tube. Looks like a small stage coupler.
Use a smaller tube size than the tube you're installing the anchor into - use a BT-20 tube scrap if you're installing the mount into a BT-50 rocket, use a BT-50 or -55 scrap for a BT-60 bird, etc etc.
Cut the tube scrap in half vertically. This will leave you with two C-shaped pieces.
Take your long elastic shock cord and tie a flat loop around one of the pieces. Apply a healthy dose of glue to the inside of the other piece, then use plastic mini-clamps to clamp the two together. Let dry 15-20 minutes until tacky.
Hand-bend the anchor so it matches the interior tube curvature of the rocket. Apply a good dose of glue to the backside and glue an inch or so down inside the tube. When it's somewhat dry, smear a glue layer over the edges of the mount and you're set.
This anchor method gives you a flatter, thinner, neater anchor on the inside of your body tube than the trifold method, which helps allow your laundry to slide out easily come ejection time.
The body tube scraps are also stronger than most cardstock or paper used for the trifolds.
Not exactly rocket science (hehe)... but it works for me. On rockets with stuffer tubes and large centering-ring motor mounts, I'll cut a slot through the top ring and epoxy the cord onto the other side, that seems to nail it down pretty well. But on most small-caliber birds, the tube-scrap anchors work OK for me.
I'm not sold on the kevlar anchors - first of all, kevlar itself is kinda expensive - and I've seen tube zippers using thread/string - I'm more confident with a 1/4" or so wide shock cord.
I agree with bobkretch, and I am sure multitudes of others - I just replace stock shock cords in kits with elastic from JoAnn's fabrics. I usually use 1/4" elastic, about 3 ' or more. The heck with these 6" cords.