thickness = 1.05; //cardboard thickness
hole= thickness * 12; //cut 
len = thickness * 40; //square len
tran= len /2 ; 
    difference() {
        square(len); //big square    
        translate( [len,tran,0] ){    
            square([hole,thickness], center = true);}
        translate( [0,tran,0] ){    
            square([hole,thickness], center = true);}
        translate( [tran,0,0] ){    
            square([thickness,hole], center = true);}
        translate( [tran,len,0] ){    
            square([thickness,hole], center = true);}
        }