Stretchy cast off for a ribbed edge

[spinmyown]

Hi
I have just finished knitting some soakers for my little one ( to cover her cloth nappy). I've knit it flat and the ribbed edge is too tight :( . I have just found the Double Cast-On method and am using it for some knitted longies i am making and found it WONDERFUL. So i will have to rip back my waists on my soakers. BUT my question is, how do you do a
Stretchy cast off for a ribbed edge???
TIA

Kylie