Unblockable Automatic PopUp

I need an unblockable popunder script working on page load and opening on a real browser window of latest versions of firefox, IE 6,7,8,9 , and Chrome. Not a dhtml layer popup. If possible, I would like this popup to be automatically closed after any amount of time that I set (must be variable), or I can set it to stay there. I need the size to be variable too. It is a similar script that some affiliate networks like cpxinteractive, paypopup etc. uses for serving popunder ads when page loads (if blocked they display pop-up/under when user click on page)... Thanks