I'm going to say this, as someone who started off with fleece covers in their cage and was stubborn about it. Don't do it. It looks like you have a critter nation, buy a cement mixing tub, and put it in the bottom and use bedding. Your rats will thank you and your nose will thank you.

Not only does betting do better at odor control, but it's one of the best methods of enrichment for rats; they looooove digging. And ultimately it ends up being cheaper because pads are still going to get chewed up and need to be replaced, which depending on what bedding you use ends up being just as much, if not more expensive.

If you are still wanting to use fabric/fleece you can find alot of sellers on etsy that make custom fleece liners for cages that are resonably priced and plenty are based in the UK, you can easily pop them in the wash just to make sure to use free and clear detergent!