It's a great idea! Agree with souffle about the grid though. I copied an idea of Vectis's and made one out of an egg box with sisal rope/string. Sisal is safe to chew - you can get it on ebay (untreated sisal string or rope). The main thing is that it is hung safely. If it's too high up they can fall out.
Is it for a tank? If it's for a cage you can just tie the sisal to the top bars.
This is the one I have in our robo's tank (it has bars on the lid). I have it hung next to a shelf so he can get in it.
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