I got Fudge the same Novy
We use megazorb and she loves it - she's currently asleep in it in fact! It's fairly stable, I asked the same question a couple of weeks ago and Nappo gave a very interesting response:
What's good for burrowing? I decided only to do that mix as a treat though as I can't keep 3 or 4 bags of stuff in the flat at once...