sharpieRadius = 12/2;
thickness = 2;

magnetThickness = 1.67;
magnetHeight = 19;
magnetWidth = 9.5;
magnetBaseThickness = 1;

wallWidth = 0.6; // for single back wall
magnetPlay = 0.4;

backThickness = thickness + magnetThickness + wallWidth + magnetPlay;
height = magnetBaseThickness*2 + magnetHeight + magnetPlay;

difference() {
    linear_extrude(height) {
        difference() {
            union() {
                circle(sharpieRadius + thickness);
                translate([-sharpieRadius - thickness, 0])
                    square([(sharpieRadius + thickness)*2, sharpieRadius + backThickness]);
            }
            circle(sharpieRadius);
        }
    }

    translate([-magnetWidth/2, sharpieRadius + thickness, magnetBaseThickness])
        cube([magnetWidth*2, magnetThickness + magnetPlay, magnetHeight + magnetPlay]);
}