What is a popunder???
Do you mean a "pop over", actually? That is, you use a <div> on the same HTML page, put the content in that <div>, and then have the <div> appear over the top of the rest of the HTML???
That's not possible to do if you are trying to put the content from an entire URL into a <div> unless you put an <iframe> inside the <div>. Unfortunately, some sites--most notably Google--will *NOT* allow themselves to be placed inside an <iframe>.
So... Explain what you mean by "popunder" and explain what the purpose is.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.