$fn=75;

// if you want a 10mm triangle use a 8mm 3 sided cylinder for the triangle 
//    and a height so small it won't really register.
// 30 mm triangle
// .5 cm height = 5

inches_to_mm = 25.4;
module my_function(){
    minkowski(){
    cylinder(d=40,h=5,$fn=3); // makes your triangle
    cylinder(d=3,h=0.00001); // rounds off the corners
} // end minkowski 
}

difference(){
    my_function();
// translate([-6, -14, 6]) rotate([0,90,0]) cube([8,24,4]); 
    union(){
        translate([-6, -11, -1]) cube([4,22,8]); 
        holes();}
}

module holes(){ 
    top_height = -10;
    translate([top_height,14,-1]) cylinder(r=.7, h=7, $fn=50);
    translate([top_height,16,-1]) cylinder(r=.7, h=7, $fn=50);
    
    translate([top_height,-14,-1]) cylinder(r=.7, h=7, $fn=50);
    translate([top_height,-16,-1]) cylinder(r=.7, h=7, $fn=50);

    translate([-top_height+8,1,-1]) cylinder(r=.7, h=7, $fn=50);
    translate([-top_height+8,-1,-1]) cylinder(r=.7, h=7, $fn=50);
}