I have a rectangular tank with bottom plates that are at ridged and valleyed arrangement so that the slopes created all drain to a single point. The floor requires vertical stiffeners at the underside to assist in keeping the floor plates from bowing from water load. I have created a sketch on a flat plane below the sloped floors and would like to extrude up to each surface to create a plate stiffener with notches that fit the sloped floor plates. Extruded boss/base does not allow multiple surfaces to extrude to? I'm sure there is another method that a novice like me has not encountered. Maybe I should post my file?

Thanks,