We had four cats until recently (three now), and we use a cement mixing tray for their litter box. It’s about 2-1/2 by 3’, and it holds plenty of litter. It’s easy to scoop, and it can accommodate four cats easily.
Available at your local hardware store, and very, very cheap in comparison to the litter boxes you will find at the pet store.
Good idea, thanks! The liter boxes are also plastic and absorb the smell. Have to replace them time and time again.