How do I code up a pay wall? (nag screen effect)

Here is an example site: http://punani.mobi

At a predetermined time after page load I want a "nag screen" to trigger and display some info and this donate button:

<a href="https://www.onebip.com/tinyurl/131ef860cca8653e7ed82a9f94aa12de" title="Mobile payment: send and receive money with your mobile phone">
<img src="http://www.onebip.com/tools/bts/btn06.gif" alt="Transfer money with your mobile phone credit" border="0"/>
</a>

I want this screen impossible or at least very difficult to get around. After the visitor pays they are allowed to resume viewing current page.

I have messed around with a few lightboxes and stripped out the cancel button, but then clicking on the black background or just disabling java defeats it and thats if it just doesn't crash. Is there a kit for what I want? or can you show me where I'm going wrong.

Many Thanks

UPDATE:

OK I've found two services but now I've got two problems. The new script given to me by tinypass.com is below but as you can see it just jumps in on pageload how do i delay it?

example site: http://punani.mobi

tinypass script:

<script type="text/javascript">
    window._tpm = window._tpm || [];
    window._tpm['paywallID'] = '53280072';
    window._tpm['trackPageview'] = true;
</script>
<script type="text/javascript" src="//code.tinypass.com/tpl/d1/tpm.js"></script>

Secondly how do I code my own paywall so that I can insert Onebips button instead? as this new solution forces me to use tinypass and as I've just found out, it doesnt even work in all browsers anyway.

Tags: Javascript, XML, CSS, HTML, Flash