Dealing this while still early in the egg is much more difficult. When I have tied and cut them, they had already exited and tried to move about, unsuccessfully. I hadn't considered scrapping the egg and transferring to a deli cup, that is smart.
Don't use a thin dental floss, as it can cut through too easily. Use a thicker brand, or some dental tape, and a slice through is less likely. That looks to be what Deb has done. We always have dental floss/tape and some tiny sharp scissors on our med station : )
I wouldn't use a rubber band for fear it would get sucked inside the body, I think tape is less likely to cause an issue in that case. I also trim the excess dental tape as close as possible to the knot.
This is by no means a foolproof process, there is still LOTS that can go wrong, but you do have a reasonable chance of success, and 1000x better than not doing anything, which is typically a death sentence for the knotted up baby.
Best of luck!