How to attach to "hollow" 3d shape

[JAnthelme]

Hi,

I would like to attach to the "walls section faces" of this structure (see arrows).

include <BOSL2/std.scad>

  diff() {
    cuboid([80, 40, 100], rounding=5, edges=[BACK+LEFT, BACK+RIGHT])
    tag("remove") cuboid([70, 30, 100 * 1.1], rounding=2);
    tag("remove") fwd(10) cuboid([60, 30, 100 * 1.1]);
  }

I tried using attachable parts but could not get it to work.

Thanks for your help.

[adrianVmariano]

Attach to the inside cube, the one that's being subtracted. You need to add the "keep" tag so your addition doesn't get subtracted away. The attachable parts feature is for objects that provide parts, or if you're writing your own object.

  diff() {
    cuboid([80, 40, 100], rounding=5, edges=[BACK+LEFT, BACK+RIGHT])
      tag("remove") cuboid([70, 30, 100 * 1.1], rounding=2)
        attach(RIGHT,BOT,inside=true) tag("keep")cyl(r1=5,r2=3,l=25,$fn=16);
    tag("remove") fwd(10) cuboid([60, 30, 100 * 1.1]);
  }