I agree. A long time ago I made one with 5 ribs 36" long and only two ribs 60" long. The ribs were all 3/4" ply and the faces were 3/32" hardboard. It was plenty rigid. I also added 3/4" spoil board, screwed into the edges, but it was pretty flat even before that.
This is a pic of my 50lb vise sitting on the table on a tape measure.
