matter.jsという物理エンジンライブラリのオブジェクトの結合について

matter.jsのオブジェクトの結合でつまづいています。

下記コードでcarオブジェクトの上に四角形を３つ固定させたいのですが、うまく固定できずに困っています。どうすればうまくできますでしょうか？

var Engine = Matter.Engine,
    Gui = Matter.Gui,
    World = Matter.World,
    Bodies = Matter.Bodies,
    Body = Matter.Body,
    Composite = Matter.Composite,
    Composites = Matter.Composites,
    Common = Matter.Common,
    Events = Matter.Events;
    MouseConstraint = Matter.MouseConstraint,
    Constraint = Matter.Constraint;


var container = document.getElementById("canvas-container");

var engine = Engine.create(container,{render: {
    options: {
      wireframes:false,
      showIds:true
    }
}});


var offset = 5;
World.addBody(engine.world, Bodies.rectangle(400, -offset, 800.5 + 2 * offset, 50.5, { isStatic: true }));
World.addBody(engine.world, Bodies.rectangle(400, 600 + offset, 800.5 + 2 * offset, 50.5, { isStatic: true }));
World.addBody(engine.world, Bodies.rectangle(800 + offset, 300, 50.5, 600.5 + 2 * offset, { isStatic: true }));
World.addBody(engine.world, Bodies.rectangle(-offset, 300, 50.5, 600.5 + 2 * offset, { isStatic: true }));

var mouseConstraint = MouseConstraint.create(engine);
World.add(engine.world, mouseConstraint);

/*    var testComp = Composite.create();
var car = Composites.car(200, 400, 100, 40 , 30);

//ここのパラメータは要調整
var testBody = Bodies.rectangle(200,400,10,10);

//carメソッドの内部を参考に追加　ここのパラメータも要調整
var axelTest = Constraint.create({
        bodyA: car.bodies[0],
        pointA: { x: 200, y:400 },
        bodyB: testBody,
        stiffness: 0
    });
Composite.addBody(car,testBody);
Composite.addConstraint(car, axelTest);

//World.add(engine.world,Composite.add(car,testBody));

World.add(engine.world, car);
//World.add(engine.world, testBody); ここで追加はしません
Engine.run(engine);


World.add(engine.world,Composite.addBody(car,testBody));
//World.add(engine.world, testBody);
*/
var xx = 120;
var yy = 170;
var width = 200;
var height = 40;
var wheelSize = 20;
var groupId = Body.nextGroupId(),
    wheelBase = 20,
    wheelAOffset = -width * 0.5 + wheelBase,
    wheelBOffset = width * 0.5 - wheelBase,
    wheelYOffset = 40;


var car = Composite.create({ label: 'Car' }),
    body = Bodies.rectangle(xx, yy, width, height, { 
        groupId: groupId, 
        friction: 0.01,
    });

var wheelA = Bodies.circle(xx + wheelAOffset, yy + wheelYOffset, wheelSize, { 
    groupId: groupId, 
    restitution: 0.5, 
    friction: 0.9,
    density: 0.01
});
            
var wheelB = Bodies.circle(xx + wheelBOffset, yy + wheelYOffset, wheelSize, { 
    groupId: groupId, 
    restitution: 0.5, 
    friction: 0.9,
    density: 0.01
});
            
var axelA = Constraint.create({
    bodyA: body,
    pointA: { x: wheelAOffset, y: wheelYOffset },
    bodyB: wheelA,
    stiffness: 0.5
});
                
var axelB = Constraint.create({
    bodyA: body,
    pointA: { x: wheelBOffset, y: wheelYOffset },
    bodyB: wheelB,
    stiffness: 0.5
});

Composite.addBody(car, body);
Composite.addBody(car, wheelA);
Composite.addBody(car, wheelB);
Composite.addConstraint(car, axelA);
Composite.addConstraint(car, axelB);


var box = Composite.create({ label: 'box' });

var underBox = Bodies.rectangle(150,150,60,25);
var leftBox = Bodies.rectangle(110,127.5,25,70);
var rightBox = Bodies.rectangle(190,127.5,25,70);


var leftLabel = Constraint.create({
    bodyA: underBox,
    pointA: { x: -20,y: 0 },
    bodyB: leftBox,
    stiffness: 0.5
});
var rightLabel = Constraint.create({
    bodyA: underBox,
    pointA: { x: 20,y: 0 },
    bodyB: rightBox,
    stiffness: 0.5
});




// World.add(engine.world,underBox);
// World.add(engine.world,leftBox);
// World.add(engine.world,rightBox);

Composite.addBody(box,underBox);
Composite.addBody(box,leftBox);
Composite.addBody(box,rightBox);
Composite.addConstraint(box,leftLabel);
Composite.addConstraint(box,rightLabel);


var carLabel = Constraint.create({
    bodyA: body,
    pointA: { x: 0,y: 10 },
    bodyB: underBox,
    stiffness: 0.5
});
var carLabel2 = Constraint.create({
    bodyA: body,
    pointA: { x: -20,y: -10 },
    bodyB: leftBox,
    stiffness: 0.5
});
var carLabel3 = Constraint.create({
    bodyA: body,
    pointA: { x: 20,y: -10 },
    bodyB: rightBox,
    stiffness: 0.5
});

Composite.addComposite(car,box);
Composite.addConstraint(car,carLabel);


World.add(engine.world,car);

Engine.run(engine);


$("#left").click(function(t){
    var carBodies = Composite.allBodies(car);
    carBodies[1].force.x = 2;  
});

<script src="http://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<script src="https://github.com/liabru/matter-js/releases/download/0.8.0-alpha/matter-0.8.0.js" type="text/javascript"></script>
<script src="http://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.7.0/underscore.js"></script>

<div id = "canvas-container"></div>
<input id = "left" type = "button" value = "left">