diff --git a/README.md b/README.md
index ab063e3f5..99b88a590 100644
--- a/README.md
+++ b/README.md
@@ -57,10 +57,7 @@ pub extern "C" fn model(args: &HashMap<String, String>) -> fj::Shape {
     let outer_edge = fj::Circle { radius: outer };
     let inner_edge = fj::Circle { radius: inner };
 
-    let footprint = fj::Difference {
-        a: outer_edge.into(),
-        b: inner_edge.into(),
-    };
+    let footprint = fj::Difference2d::from_objects(outer_edge.into(), inner_edge.into());
 
     let spacer = fj::Sweep {
         shape: footprint.into(),
diff --git a/fj/src/shape_2d.rs b/fj/src/shape_2d.rs
index 1e1dab5ba..64777fd9e 100644
--- a/fj/src/shape_2d.rs
+++ b/fj/src/shape_2d.rs
@@ -41,15 +41,29 @@ impl From<Circle> for Shape2d {
 #[repr(C)]
 pub struct Difference2d {
     /// The original shape
-    pub a: Shape2d,
+    a: Shape2d,
 
     /// The shape being subtracted
-    pub b: Shape2d,
+    b: Shape2d,
+}
+
+impl Difference2d {
+    pub fn from_objects(a: Shape2d, b: Shape2d) -> Self {
+        Self { a, b }
+    }
+
+    pub fn a(&self) -> &Shape2d {
+        &self.a
+    }
+
+    pub fn b(&self) -> &Shape2d {
+        &self.b
+    }
 }
 
 impl From<Difference2d> for Shape {
     fn from(shape: Difference2d) -> Self {
-        Self::Shape2d(Shape2d::Difference(Box::new(shape)))
+        Self::Shape2d(shape.into())
     }
 }
 
diff --git a/models/spacer/src/lib.rs b/models/spacer/src/lib.rs
index 3a84e8e98..6e83ca0e4 100644
--- a/models/spacer/src/lib.rs
+++ b/models/spacer/src/lib.rs
@@ -21,10 +21,7 @@ pub extern "C" fn model(args: &HashMap<String, String>) -> fj::Shape {
     let outer_edge = fj::Circle { radius: outer };
     let inner_edge = fj::Circle { radius: inner };
 
-    let footprint = fj::Difference2d {
-        a: outer_edge.into(),
-        b: inner_edge.into(),
-    };
+    let footprint = fj::Difference2d::from_objects(outer_edge.into(), inner_edge.into());
 
     let spacer = fj::Sweep {
         shape: footprint.into(),
diff --git a/models/star/src/lib.rs b/models/star/src/lib.rs
index 8bfb958ad..c8eeaa9f2 100644
--- a/models/star/src/lib.rs
+++ b/models/star/src/lib.rs
@@ -53,10 +53,7 @@ pub extern "C" fn model(args: &HashMap<String, String>) -> fj::Shape {
     let outer = fj::Sketch::from_points(outer);
     let inner = fj::Sketch::from_points(inner);
 
-    let footprint = fj::Difference2d {
-        a: outer.into(),
-        b: inner.into(),
-    };
+    let footprint = fj::Difference2d::from_objects(outer.into(), inner.into());
 
     let star = fj::Sweep {
         shape: footprint.into(),
diff --git a/src/kernel/shapes/difference_2d.rs b/src/kernel/shapes/difference_2d.rs
index 556a91a95..81233fc0c 100644
--- a/src/kernel/shapes/difference_2d.rs
+++ b/src/kernel/shapes/difference_2d.rs
@@ -22,7 +22,7 @@ impl ToShape for fj::Difference2d {
 
         let mut shape = Shape::new();
 
-        let [mut a, mut b] = [&self.a, &self.b]
+        let [mut a, mut b] = [&self.a(), &self.b()]
             .map(|shape| shape.to_shape(tolerance, debug_info));
 
         for shape in [&mut a, &mut b] {
@@ -105,6 +105,6 @@ impl ToShape for fj::Difference2d {
         // This is a conservative estimate of the bounding box: It's never going
         // to be bigger than the bounding box of the original shape that another
         // is being subtracted from.
-        self.a.bounding_volume()
+        self.a().bounding_volume()
     }
 }