infty = 100;
eps = 0.01;

module spacer() {
	difference() {
		cylinder(r = 33/2, h = 5);
		translate([0, 0, -eps])
			cylinder(r = 15/2, h = 5 + 2*eps);
	}
}

module inner() {
	difference() {
		translate([-21/2, -21/2, 0])
			cube([21, 21, 21]);
		// hole
		translate([0, 0, -eps])
			cylinder(r = 11.5/2, h = 21 + 2*eps);
		// hole - moiddle ring
		translate([0, 0, 9])
			cylinder(r = 12.5/2, h = 3);
		// bottom cuts
		for (x = [-1, 1]) scale([x, 1, 1])
		translate([21/2-1, -infty/2, 0])
		rotate([0, 45, 0])
			cube(infty);
		// bottom cuts
		for (x = [-1, 1]) scale([x, 1, 1])
		translate([21/2, -infty/2, 21-1])
		rotate([0, -45, 0])
			cube(infty);
	}
}
	
//  spacer();

inner();