Is the cat's litter making its way out of the box? It could be stuck to their feet, or they may be flicking it out of the box out of spite. Whatever the case may be the Litter catcher trey will help reduce the amount of litter that travels away from the box.
Print 4 corners and just use those, or print connector pieces to extend it.
Filament: PLA, PETG, ABS
Infill: 30-50%
Wall thickness 3-6 loops
Scale to a printable size for your bed size