With my robos I like to provide 3 types of floor covering (spoilt or what?
) I divide the bottom of the tank in 3 by woodshavings, wood pellets & then a 'grassy mat' - which is flat so the wheel stays up & doesn't get bits of wood stuff in it.
Then I provide the 'safe bed' shredded cloth stuff for them in their house(s) but they generally just dig a 'butt grove' in the wood stuff & curl up like little buddhas