diff --git a/examples/abcdef.rs b/examples/abcdef.rs
index 1a14c9d..b4e8699 100644
--- a/examples/abcdef.rs
+++ b/examples/abcdef.rs
@@ -11,7 +11,7 @@ fn main() {
             Box::new(ks::kernel::Matern52::new(0.5, 1.0)),
         ];
 
-        model.add_item(player, Box::new(kernel));
+        model.add_item(player, kernel);
     }
 
     model.observe(&["A"], &["B"], 0.0);
diff --git a/examples/kickscore-basics.rs b/examples/kickscore-basics.rs
index 53bb2ad..24102e2 100644
--- a/examples/kickscore-basics.rs
+++ b/examples/kickscore-basics.rs
@@ -21,9 +21,9 @@ fn main() {
     ];
 
     // Now we are ready to add the items in the model.
-    model.add_item("Spike", Box::new(k_spike));
-    model.add_item("Tom", Box::new(k_tom));
-    model.add_item("Jerry", Box::new(k_jerry));
+    model.add_item("Spike", k_spike);
+    model.add_item("Tom", k_tom);
+    model.add_item("Jerry", k_jerry);
 
     // At first, Jerry beats Tom a couple of times.
     model.observe(&["Jerry"], &["Tom"], 0.0);
diff --git a/examples/nba-history.rs b/examples/nba-history.rs
index 9e70778..7e2cc66 100644
--- a/examples/nba-history.rs
+++ b/examples/nba-history.rs
@@ -62,7 +62,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             Box::new(ks::kernel::Matern32::new(0.138, 1.753 * seconds_in_year)),
         ];
 
-        model.add_item(&team, Box::new(kernel));
+        model.add_item(&team, kernel);
     }
 
     for (winner, loser, t) in observations {
diff --git a/src/fitter/recursive.rs b/src/fitter/recursive.rs
index 40972d5..2978916 100644
--- a/src/fitter/recursive.rs
+++ b/src/fitter/recursive.rs
@@ -7,9 +7,9 @@ use crate::kernel::Kernel;
 
 use super::Fitter;
 
-pub struct Recursive {
+pub struct Recursive<K> {
     ts_new: Vec<f64>,
-    kernel: Box<dyn Kernel>,
+    kernel: K,
     ts: Vec<f64>,
     ms: Vec<f64>,
     vs: Vec<f64>,
@@ -28,8 +28,8 @@ pub struct Recursive {
     p_s: Vec<Array2<f64>>,
 }
 
-impl Recursive {
-    pub fn new(kernel: Box<dyn Kernel>) -> Self {
+impl<K: Kernel> Recursive<K> {
+    pub fn new(kernel: K) -> Self {
         let m = kernel.order();
         let h = kernel.measurement_vector();
 
@@ -56,7 +56,7 @@ impl Recursive {
     }
 }
 
-impl fmt::Debug for Recursive {
+impl<K: Kernel> fmt::Debug for Recursive<K> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("RecursiveFitter")
             .field("ts_new", &self.ts_new)
@@ -80,7 +80,7 @@ impl fmt::Debug for Recursive {
     }
 }
 
-impl Fitter for Recursive {
+impl<K: Kernel> Fitter for Recursive<K> {
     fn add_sample(&mut self, t: f64) -> usize {
         let idx = self.ts.len() + self.ts_new.len();
 
diff --git a/src/model/binary.rs b/src/model/binary.rs
index 8eeb770..443d627 100644
--- a/src/model/binary.rs
+++ b/src/model/binary.rs
@@ -42,7 +42,7 @@ impl Binary {
         }
     }
 
-    pub fn add_item(&mut self, name: &str, kernel: Box<dyn Kernel>) {
+    pub fn add_item<K: Kernel + 'static>(&mut self, name: &str, kernel: K) {
         if self.storage.contains_key(name) {
             panic!("item '{}' already added", name);
         }
diff --git a/src/model/difference.rs b/src/model/difference.rs
index c62a6bd..f7f0c28 100644
--- a/src/model/difference.rs
+++ b/src/model/difference.rs
@@ -31,7 +31,7 @@ impl DifferenceModel {
         }
     }
 
-    pub fn add_item(&mut self, name: &str, kernel: Box<dyn Kernel>) {
+    pub fn add_item<K: Kernel + 'static>(&mut self, name: &str, kernel: K) {
         if self.storage.contains_key(name) {
             panic!("item '{}' already added", name);
         }
diff --git a/src/model/ternary.rs b/src/model/ternary.rs
index 8a7d970..a44858a 100644
--- a/src/model/ternary.rs
+++ b/src/model/ternary.rs
@@ -39,7 +39,7 @@ impl TernaryModel {
         }
     }
 
-    pub fn add_item(&mut self, name: &str, kernel: Box<dyn Kernel>) {
+    pub fn add_item<K: Kernel + 'static>(&mut self, name: &str, kernel: K) {
         if self.storage.contains_key(name) {
             panic!("item '{}' already added", name);
         }
diff --git a/tests/binary-1.rs b/tests/binary-1.rs
index 68079e8..badcb2b 100644
--- a/tests/binary-1.rs
+++ b/tests/binary-1.rs
@@ -9,8 +9,8 @@ fn binary_1() {
     let k_audrey = ks::kernel::Matern52::new(1.0, 2.0);
     let k_benjamin = ks::kernel::Matern52::new(1.0, 2.0);
 
-    model.add_item("audrey", Box::new(k_audrey));
-    model.add_item("benjamin", Box::new(k_benjamin));
+    model.add_item("audrey", k_audrey);
+    model.add_item("benjamin", k_benjamin);
 
     model.observe(&["audrey"], &["benjamin"], 0.0);
     model.observe(&["audrey"], &["benjamin"], 1.0);
diff --git a/tests/kickscore-basics.rs b/tests/kickscore-basics.rs
index e793656..0fd0956 100644
--- a/tests/kickscore-basics.rs
+++ b/tests/kickscore-basics.rs
@@ -16,9 +16,9 @@ fn kickscore_basic() {
         Box::new(ks::kernel::Matern52::new(0.5, 1.0)),
     ];
 
-    model.add_item("Spike", Box::new(k_spike));
-    model.add_item("Tom", Box::new(k_tom));
-    model.add_item("Jerry", Box::new(k_jerry));
+    model.add_item("Spike", k_spike);
+    model.add_item("Tom", k_tom);
+    model.add_item("Jerry", k_jerry);
 
     model.observe(&["Jerry"], &["Tom"], 0.0);
     model.observe(&["Jerry"], &["Tom"], 0.9);
diff --git a/tests/nba-history.rs b/tests/nba-history.rs
index 96da27f..112a7f1 100644
--- a/tests/nba-history.rs
+++ b/tests/nba-history.rs
@@ -66,7 +66,7 @@ fn nba_history() {
             Box::new(ks::kernel::Matern32::new(0.138, 1.753 * seconds_in_year)),
         ];
 
-        model.add_item(&team, Box::new(kernel));
+        model.add_item(&team, kernel);
     }
 
     for (winner, loser, t) in observations {