$fn=180; // More precise shapes 

$cote = 40; //big square lenght
$kerf = 0.25;
$deep = 10; // lenght of the small squares 
$lenght = 3-$kerf; // lenght of the small squares



difference() {  
square([$cote, $cote]);
translate([$cote/2-1.5, 0]) // down square
square([$lenght,$deep]);
    
translate([$cote/2-1.5, $cote-$deep]) // up square
square([$lenght,$deep]);

translate([0, $cote/2-1.5]) // left square 
square([$deep,$lenght]);
    
translate([$cote-$deep, $cote/2-1.5]) // right square 
square([$deep,$lenght]);}