A Kitty Toybox

My feline furbabies are indoor cats, which means they do not go outside unless they're in a travel box or on a leash. For this reason I am always looking for new ways to keep them entertained and stimulated.
Buying cat toys on the web (or in a shop) can cost you an arm and a leg, so if I can I make my own. A while ago I saw a toy on the internet that was literally a box with holes in, and here is my version.
It was a cake box and its not completely opaque. My son cut holes in the sides and in the lid that are big enough for the cats to get a whole leg into, and I now put a catnip-smelling toy into the box.
Because they can see it, the challenge is to get to it and get it out.
Diva, Turk and Greebo are the most successful with this toy, Magic is not at all interested and Minx can't work out how to get to it through the holes.