Trouble understanding diff and how to remove the duplicate code

[rvt]

Hey,

I want to create a box with some screw holes. For this I make a box in a box.. and use a diff to remove the holes. I am properly over thinking this but I cannot get the diff right..

Without the first first part, the hole and the 'support' are holes and support is on the outside of the box, but not on the inside.
Without the second part the support is on the inside, but not on the outside, and there is no hole.
If I use both, the system does what I want, but it feel incorrect.

What would be the correct way to approach this?

This how I want it to function:

include <BOSL2/std.scad>

$fn = $preview ? 32 : 256;


DIST=23;

SCREWS=[45,135,225, 315];

diff()
cuboid([35,35,6], anchor=BOTTOM) {

  diff("rem2",$tag="remove") {
    position(BOTTOM)
    up(1)
    cuboid([33,33,6], anchor=BOTTOM);
    
    /* Duplicated below, how to avoid?? */
    for (n = SCREWS) {
      zrot(n)
      left(DIST) {
        tag("rem2")
        diff(remove="rem3")
          position(BOTTOM) {
            cyl(d=4,l=6, anchor=BOTTOM);
            cyl(d=6,l=3, anchor=BOTTOM);
          }
        tag("remove") cyl(d=2,l=6*3, center=true);
      }
    }
  }
   for (n = SCREWS) {
    zrot(n)
    left(DIST) {
      tag("rem2")
      diff(remove="rem3")
        position(BOTTOM) {
          cyl(d=4,l=6, anchor=BOTTOM);
          cyl(d=6,l=3, anchor=BOTTOM);
        }
      tag("remove") cyl(d=2,l=6*3, center=true);
    }
  }
}