diff --git a/cad/openscad/owen_focus_ring.scad b/cad/openscad/owen_focus_ring.scad
index 20e85fb..8fe4644 100644
--- a/cad/openscad/owen_focus_ring.scad
+++ b/cad/openscad/owen_focus_ring.scad
@@ -2,7 +2,7 @@ $fn = 500;
 ir = 65 / 2;
 er = ir + 1.5;
 
-linear_extrude(30) {
+module main() {
     difference() {
         circle(er);
         circle(ir);
@@ -10,4 +10,14 @@ linear_extrude(30) {
     for(i = [0:3:359]) {
         translate([er*sin(i), er*cos(i)]) circle(r=1.5);
     }
+}
+
+difference() {
+    linear_extrude(30) {
+        difference() {
+            main();
+            translate([0, -0.5]) square([er + 1.5, 1], center=false);
+        }
+    }
+    for(i = [0:120:240]) translate([0, 0, 17])  rotate(i, [0, 0, 1]) rotate(90, [0, 1, 0]) cylinder(h=er+1.5, r=1);
 }
\ No newline at end of file