I use 2 corner littler boxes with the grate in them and one small square plastic box. Three boxes for four ratties - I am going to try to pare that down to just the two corner ones though...not sure how they'll like that!!
I use a corner box with a grate over it. Easy to clean and they use it!! On the other level I just use an ordinary cardboard box with the same substrate that's in the corner box (Yesterday's News). When it gets gross I just toss the whole thing and replace it with the same.