foo.html

<template name="windowFrame">
  <div class="framing-classes">
    {{> Template.dynamic
        template=contentTemplate
        data=contentData}}
  </div>
<div>

<template name="myWindow">
  <h1>{{title}}</h1>
  <p>Various stuff and whatnot</p>
</template>

foo.js

function makeWindow(contentTemplate, data) {
  Blaze.renderWithData(
    Template.windowFrame, 
    {
      contentTemplate: contentTemplate, 
      contentData: data
    },
    document.body);
}

// render Template.windowFrame with Template.myWindow inside it:

makeWindow("myWindow", {title:"Fancy Title"});