Have a piece of 1/2" plywood cut to 37.25" by 17".
Following the drawing at simbotics.org/kitbot, drill clearance holes for the chassis nuts, or cut out the indicated area in the corners of the baseboard.
Drill mounting holes for screws to mount the board to the chassis. Either 1/4-20 (17/64" clearance hole) or 10-32 (13/64" clearance hole) screws may be used.
Align the baseboard with the bottom of the chassis and place a screw with washer through each of the mounting holes. Tighten the screws down with the appropriately sized nylock nut.