Colt fragging question

[Bob I]

I have what I will call a Colt Coral for now (see picture on my site) This thing grows like mad, and I have been trying to frag it. The unfortunate thing is that I cannot get it to anchor. The first we tried was an elastic band. It just cut through, and made two frags. I then got some rubble, and tried to wedge it. It just wiggles out. Then I tried putting it on top of a piece of rubble, and piling rubble on top. It just squeezes out, and floats free. I have recently taken a small plastic container, and put rubble in that, and the frag in between. The frag is still there, but I have no idea if it is anchoring. Does anyone else have any ideas? The stuff drives me insane.