I found an easy and secure way to create custom web apps: using Firefox and the Greasemonkey extension. In Greasemonkey, it's enough to create a user script like this (I've created for Asana):
// ==UserScript==
// @name Asana
// @namespace app.asana.com
// @include https://app.asana.com/*
// @grant none
// @version 1
// ==/UserScript==
window.Unity = external.getUnityObject(1);
Unity.init({ name: 'Asana',
domain: 'app.asana.com',
homepage: 'https://app.asana.com/',
iconUrl: 'http://asana.com/assets/FluidIcon.png'
});
This script simply enable the Unity integration, but you can add other features writing more code.