Add cmount_rear_cap

This is a super-simple c-mount cap

models/cmount_rear_cap/cmount_rear_cap.scad

@@ -0,0 +1,13 @@
+module cmount_rear_cap () {
+    $fn = 200;
+    ID = 25.26;
+    OD = 30;
+    H = 6.55;
+    T = 2.15;
+    difference () {
+        cylinder(r = OD / 2, h = H, center = true);
+        translate([0, 0, T]) cylinder(r = ID / 2, h = H, center = true);
+    }
+}
+
+cmount_rear_cap();