That is exactly what I made. It was kind of a pain to make the flap work right because of the nature of the coroplast. I also need stronger magnets to get the top to pick it up as far as it needs to. Until I get the magnets, I have just cut a piece of coroplast the size of the bottom and simply laid it in the tray as a liner. Works perfect to protect the bottom of the tray from cat claws tearing up the cardboard allowing it to get wet, stinky, and fall apart.
With 3 cats, those refills do not last 30 days! Not at my house anyway, and I have 2 boxes. I think I changed it at about 3 weeks.
Great box, works without a hitch so far.