Shock cords--the nose cone and body are zipping apart at a fast, but decreasing velocity. The energy that the shock cord has to absorb is proportional to the square of the velocity. If you want things to stay together, you need to do one of three things:

1--slow down the flying objects more quickly
2--reduce the mass of the objects
3--wait for friction to do its thing and slow them down on its own (i.e. longer shock cord)

Elastic has the nice effect of slowing things down quickly as it stretches, but has the unpleasant effect of snap-back. I strongly prefer chain-stitched (or monkey braided) Kevlar, as it greatly reduces the force on the system while the knots are unfurling, and doesn't snap back. My R & D report at last NARAM measured this and found that the chain-stitched Kevlar took something like 2/3 of the total energy out of the system before actually yanking on the nose cone tether/Kevlar attachment points.

But what do I know? I usually taper my fins