Create a jquery/javascript scroll modal set by session cokie

I need a very simple task with a quick turnover today if possible.. I need a modal box to work similar to this example http://www.g2k.nl. The difference being once scrolled off the page and the body is enabled to scroll a cookie is set to last the session so the modal will not show again until the cookie expires. I will require a single working page example with dummy text for me to implement into the website. The website is a concrete5 site using jquery but plain javascript would be OK.