module rounded_square(dimensions, radius, center=true) {
  minkowski(){
    square(dimensions - 2 * [radius,radius], center=center);
    circle(radius);
  }
}

CASE_WIDTH = 245;
CASE_HEIGHT = 92;
CASE_THICKNESS = 6.0;
SCREW_DIAMETER = 3.0;
SCREW_OFFSET = 4;
// solution and simplification of 
// 2*(INNER_RADIUS+CASE_THICKNESS)^2 = (INNER_RADIUS+OUTER_RADIUS+sqrt(2 * SCREW_OFFSET ^ 2))^2
// INNER_RADIUS = sqrt(2) * (2 * SCREW_OFFSET - CASE_THICKNESS) + 3 * SCREW_OFFSET - 2 * CASE_THICKNESS;
INNER_RADIUS = (2 + sqrt(2)) * (2 * SCREW_OFFSET - CASE_THICKNESS) - SCREW_OFFSET;

echo("INNER_RADIUS=", INNER_RADIUS);


difference() {

  rounded_square([245,92], SCREW_OFFSET);
  translate([+(CASE_WIDTH/2 - SCREW_OFFSET), +(CASE_HEIGHT/2 - SCREW_OFFSET)]) circle(SCREW_DIAMETER / 2);
  translate([+(CASE_WIDTH/2 - SCREW_OFFSET), -(CASE_HEIGHT/2 - SCREW_OFFSET)]) circle(SCREW_DIAMETER / 2);
  translate([-(CASE_WIDTH/2 - SCREW_OFFSET), +(CASE_HEIGHT/2 - SCREW_OFFSET)]) circle(SCREW_DIAMETER / 2);
  translate([-(CASE_WIDTH/2 - SCREW_OFFSET), -(CASE_HEIGHT/2 - SCREW_OFFSET)]) circle(SCREW_DIAMETER / 2);
  rounded_square([245 - 2 * CASE_THICKNESS,92 - 2 * CASE_THICKNESS], INNER_RADIUS);
}