Sorry, no. The way the 10" box fits under the seat, it doesn't stick out far past the front of the 60 seat correct? Meaning if someone sits there, there is no interference to where they put their feet. Well, if you make a 12" box, it would stick out just a bit from the underside of the seat, in return making a slight ledge extending from the front side of the seat on the floor. Also it would be a bit longer of a box that extends to the edge of the 40 seat, not going underneath the 40 seat. I know this is hard to word. Sorry, DD ST