This is something I was looking into a few years back, and the sat up with your new-old one is one I was thinking about making myself.
I never thought of using egg cartons myself, there was a blog I read that said to use leaves for them to hide into, but I like the idea of egg cartons as there is plenty of them going around.
I'll be checking back to see the results, are you having both boxes on to see which collects the most?