Old or broken tiles (Blackpool FY1)

I’m looking for waste thrown away tiles to line a cardboard litter tray. It doesn’t matter if they’re chipped or broken into bits. You’re supposed to throw out the litter tray and replace it every month but they cost £54 for a pack of 3 which is crazy. They’re the only ones that will fit the electric raking litter box I was given by somebody. I can’t use it without a bottom tray. Possibly some other material would work - acrylic sheets or corex/corrugated plastic. So if anyone is throwing away fridge shelves (acrylic) or campaign signs/House for sale signs (corrugated plastic) I’d be grateful for those. Or if anyone has any other ideas for a material to make the cardboard tray waterproof, I’m open to suggestions and I’d be grateful.