small_distance = 0.01;
width = 250;
height = 210;
wall_thickness = 0.8 + small_distance;
wall_height = 20;

linear_extrude(wall_thickness)
square([width,height], center=true);

linear_extrude(wall_height)
difference(){
    square([width,height], center=true);
    square([width-2*wall_thickness,height-2*wall_thickness], center=true);
}