I don't think fleece material would be good because it would get dirty to fast and hold too much bacteria.I'm thinking maybe straw? And of course it needs to be changed regularly.
Have you checked out nesting boxes in stores or online, and then you will have an idea of how to model one yourself?
Do go to the breeding section here and read all you can. There is alot of good info there and you need to be prepared
I know Vincent's Q Tip just had babies so maybe you could PM him about it?
Good luck,and do your research BEFORE they breed.