Edit in JSFiddle

var world;
var sw;
var sh;
var renderer;
var stage;

var fixedTimeStep = 1 / 60;
var colorList = [
	0xf64c4c,
  0xf6844c,
  0xd0f64c,
  0x4cc8f6
]
var itemList = [];

function main() {
	sw = window.innerWidth;
  sh = window.innerHeight;
  
	renderer = new PIXI.CanvasRenderer({
  	width: sw,
    height: sh,
	  autoResize:true
  });
  renderer.backgroundColor = 0xffffff;
  document.getElementById('stage').appendChild(renderer.view);
  stage = new PIXI.Container();
  
  world = new p2.World({
  	gravity: [0, -9.82]
  });
  
  world.on('beginContact', function() {
  	
  })
  
  var outerData2 = analyzePathData2(outerPathData2, 0.8, outerPathWidth, outerPathHeight);
  var innerData2 = analyzePathData2(innerPathData2, 0.8, innerPathWidth, innerPathHeight);
  createConcave(sw / 2, sh / 2, outerData2.p2Path, outerData2.path, 'outerBody');
  createConcave(sw / 2, sh / 2, innerData2.p2Path, innerData2.path, 'innerBody');
  
  createWall(sw / 2 - 200, sh / 2, 10, 400, 0);
  createWall(sw / 2 + 200, sh / 2, 10, 400, 0);
  createWall(sw / 2, sh / 2 + 200, 400, 10, 0);
  
  window.addEventListener('resize', resize);
  resize();
  
  requestAnimationFrame(animate);
}

function animate(time) {
	requestAnimationFrame(animate);
  world.step(fixedTimeStep);
  render();
}

function render() {
  createBall();
  
  for (var i = itemList.length - 1; i >= 0; i--) {
  	var graphic = itemList[i];
    if (graphic.body.world && graphic.shape.type == p2.Shape.CIRCLE) {
    	var x = p2ToPixiX(graphic.body.position[0]);
    	var y = p2ToPixiY(graphic.body.position[1]);
      if (y > sh + 50) {
      	world.removeBody(graphic.body);
        stage.removeChild(graphic);
        itemList.splice(i, 1);
        graphic.destroy();
      } else {
      	graphic.x = x;
        graphic.y = y;
      }
    }
  }
  
  renderer.render(stage);
}

function createBall() {
  var randX = sw / 2 + (Math.random() * 100 - 50);
  var randY = Math.random() * 50 - 50;
  var xx = pixiToP2X(randX);
  var yy = pixiToP2Y(randY);
  var radius = 10;
  var ballShape = new p2.Circle({radius:pixiToP2Value(radius)});
  var ballBody = new p2.Body({
  	mass:1.0, 
    position: [xx, yy]
  });
  var colorID = Math.floor(Math.random() * colorList.length);
  var color = colorList[colorID];
  ballBody.color = color;
  ballBody.addShape(ballShape);
  world.addBody(ballBody);
  
  var graphic = new PIXI.Graphics();
  graphic.beginFill(color);
  graphic.drawCircle(0, 0, radius);
  graphic.endFill();
  graphic.pivot.set(0.5, 0.5);
  graphic.x = randX;
  graphic.y = randY;
  stage.addChild(graphic);
  graphic.body = ballBody;
  graphic.shape = ballShape;
  itemList.push(graphic);
}

function createWall(x, y, w, h, rotation) {
	var p2X = pixiToP2X(x);
  var p2Y = pixiToP2Y(y);
  var p2W = pixiToP2Value(w);
  var p2H = pixiToP2Value(h);
  var shape = new p2.Box({
  	width:p2W, 
    height:p2H,
  });
  var body = new p2.Body({
  	mass:0,
    position:[p2X, p2Y],
    angle:-rotation
  });
  body.addShape(shape);
  world.addBody(body);
  
  /*
  var graphic = new PIXI.Graphics();
  graphic.beginFill(0x666666);
  console.log('x:' + x + ' y:' + y + ' w:' + w + ' h:' +h);
  graphic.drawRect(-w/2, -h/2, w, h);
  //graphic.drawCircle(0, 0, 10);
  graphic.endFill();
  graphic.pivot.set(0.5, 0.5);
  graphic.x = x;
  graphic.y = y;
  graphic.rotation = rotation;
  stage.addChild(graphic);
  graphic.body = body;
  graphic.shape = shape;
  */
}

function createConcave(x, y, p2Path, path, name) {
	var p2X = pixiToP2X(x);
  var p2Y = pixiToP2Y(y);
  var body = new p2.Body({
  	mass:0,
    position:[p2X, p2Y],
  });
  body.fromPolygon(p2Path);
  body.name = name;
  for (var i = 0, len = body.shapes.length; i < len; i++) {
		var shape = body.shapes[i];
    //shape.sensor = true;
  }
  world.addBody(body);
  
  /*
  var graphics = new PIXI.Graphics();
  graphics.beginFill(0x666666);
  graphics.moveTo(path[0][0], path[0][1]);
  for (var i = 1, len = path.length; i < len; i++) {
  	graphics.lineTo(path[i][0],path[i][1]);
  }
  graphics.endFill();
  graphics.x = x;
  graphics.y = y;
  stage.addChild(graphics);
  */
}

function analyzePathData2(pathData, scale, width, height) {
	var points = pathData.split(' ');
  var result = {};
  result.path = [];
  result.p2Path = [];
  var wid = width * scale;
  var hei = height * scale;
  for (var i = 0, len = points.length; i < len; i++) {
  	var pt = points[i];
    var point = pt.split(',');
    var x = point[0] * scale - wid/2;
    var y = point[1] * scale - hei/2;
    var p2X = pixiToP2X(x);
    var p2Y = pixiToP2Y(y);
    result.path.push([x, y]);
    result.p2Path.push([p2X, p2Y]);
  }
  return result;
}

function resize(evt) {
	sw = window.innerWidth;
  sh = window.innerHeight;
}

 // p2→pixi
function p2ToPixiX(p2X) {
  return p2X * 100;
}
function p2ToPixiY(p2Y) {
  return -(p2Y * 100);
}
function p2ToPixiValue(p2Value) {
  return p2Value * 100;
}

// pixi→p2
function pixiToP2X(pixiX) {
  return pixiX / 100;
}
function pixiToP2Y(pixiY) {
  return -(pixiY / 100);
}
function pixiToP2Value(pixiValue) {
  return pixiValue / 100;
}


var innerPathWidth = 116;
var innerPathHeight = 112;
var innerPathData2 = "62.5,0.5 33.5,9.5 12.5,29.5 0.5,53.5 0.5,76.5 10.5,97.5 27.5,107.5 45.5,112.5 67.5,108.5 87.5,96.5 104.5,76.5 111.5,56.5 111.5,37.5 103.5,20.5 91.5,8.5 77.5,1.5";

var outerPathWidth = 346;
var outerPathHeight = 340;
var outerPathData2 = "144.5,9.5 133.5,78.5 169.5,65.5 202.5,62.5 232.5,70.5 254.5,85.5 272.5,109.5 282.5,138.5 282.5,169.5 272.5,202.5 253.5,232.5 229.5,253.5 201.5,269.5 169.5,278.5 136.5,278.5 109.5,269.5 84.5,249.5 68.5,220.5 63.5,188.5 87.5,38.5 46.5,75.5 20.5,117.5 5.5,158.5 0.5,193.5 4.5,229.5 14.5,263.5 34.5,292.5 58.5,314.5 88.5,330.5 121.5,338.5 165.5,339.5 207.5,328.5 245.5,310.5 273.5,291.5 314.5,338.5 345.5,163.5 345.5,120.5 331.5,76.5 309.5,43.5 278.5,18.5 238.5,3.5 205.5,-0.5 176.5,1.5";

main();

<div id="stage"></div>

html, body {
  margin: 0;
  padding: 0;
  width:100%;
  height:100%;
  overflow: hidden;
}