difference() {
    union() {
        // base object
        cube([40, 28, 1]){}
            
        // four corner nubbins
        translate([5, 0, 0]){
            cylinder(1, 5, 5){}
        }
        translate([5, 28, 0]){
            cylinder(1, 5, 5){}
        }
        translate([35, 0, 0]){
            cylinder(1, 5, 5){}
        }
        translate([35, 28, 0]){
            cylinder(1, 5, 5){}
        }
    }

    // ring hook hole
    translate([5.5, 14, -1]){
        cylinder(3, 4, 4){}
    }
    
    // thread catchers
    translate([0, 2, -1]) {
        rotate([0, 0, 35]){
            cube([10, .25, 3]){}
        }
    }
    translate([8.25, 20, -1]) {
        rotate([0, 0, 145]){
            cube([10, .25, 3]){}
        }
    }
    translate([40.25, 2, -1]) {
        rotate([0, 0, 145]){
            cube([10, .25, 3]){}
        }
    }
    translate([32.25, 20, -1]) {
        rotate([0, 0, 35]){
            cube([10, .25, 3]){}
        }
    }
}