This is a midwinter picture so ignore the mess!
As you can see I don’t have a hutch in there now but this is no different in terms of flooring than when it was a hutch with c&c grid run attached.
The coloured floor coverings you can see are old oilcloth tablecloths; one of which I did originally have on my dining table. It goes along under the cage to the right of the picture also. That is all that originally lined the shed floor.
They do have black correx within the cage now but that’s only because we had some kicking around after we needed some floor protection during some work we did on our house year ago. Thought I’d use it in the cage rather than chuck it away - had we not bought it for the house work, I would not have bothered buying it just for use in the cages as the oilcloth tablecloths work perfectly well. It is 2mm. It is completely flattened with no corrugation left given it has been stood on.
You can get also 4mm thick correx which is recommended for inside a cage.
I will say that having the correx within the cage and having created a little upstand does mean the shavings stay inside the cage now. Previously, and obviously, it fell out of the cage sides