1  rows = 4;
 2  cols = 5;
 3  border = 3;
 4  
 5  dx = .76*25.4;
 6  dy = .76*25.4;
 7  
 8  sx = .551*25.4;
 9  sy = .551*25.4;
10  
11  hdia = 5.5; // note: m2.5 nylon screw heads are 4.5mm dia
12  linear_extrude(.06*25.4)
13  difference() {
14      translate([-border-sx/2,-border-sx/2])
15      square([dx*(rows-1) + sx + 2*border, dy*(cols-1) + sy +2*border]);
16  
17      for(r=[0:1:rows-1]) {
18          for(c=[0:1:cols-1]) {
19              translate([dx*r, dy*c])
20              square([sx,sy], center=true);
21          }
22  
23          translate([dx/2, dy/2])
24          circle(d=hdia, $fn=36);
25          translate([dx*(2 +1/2), dy/2])
26          circle(d=hdia, $fn=36);
27      }
28  
29  }