iron-router-not-found.html

<template name="layout">
  <p>Current page = {{currentPage}}</p>
  <p><a href="/">Main</a></p>
  <p><a href="/notfound">Not found</a></p>
  {{yield}}
</template>
<template name="main">
  <p>Main page</p>
</template>
<template name="not_found">
  <p>Not found page</p>
</template>

iron-router-not-found.js

Router.configure({
  layoutTemplate: 'layout',
  notFoundTemplate: 'not_found',
});
Router.route('main', {path: '/'});

if (Meteor.isClient) {
  Template.layout.helpers({
    currentPage: function() {
      if (!Router.current())
        return '<not-found>';
      return Router.current().template;
    }
  });
}

smart.json

{
  "packages": {
    "iron-router": {}
  }
}

smart.lock

{
  "meteor": {},
  "dependencies": {
    "basePackages": {
      "iron-router": {}
    },
    "packages": {
      "iron-router": {
        "git": "https://github.com/EventedMind/iron-router.git",
        "tag": "v0.6.2",
        "commit": "ce56e58b45624dce992490f3a1c10f10d3948bae"
      }
    }
  }
}