As seen below, I'm using 120x90 ads. The code should be placed as follows:

HTML
/* BODY HERE */

$body = <<<EOF

<div align="center">
<script type="text/javascript">
<!--
google_ad_client = "pub-3411120354601471";
/* UBB 120x90 Link Unit */
google_ad_slot = "7692017375";
google_ad_width = "120";
google_ad_height = "90";
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</div>

EOF;

In control panel, the cache time should be 0 (zero) for this custom island.

GangsterBB.NET (Ver. 7.6.1.1)
PHP Version 5.6.40 / MySQL 5.7.23-23 (was 5.6.41-84.1) / Apache 2.4.54
2007 Content Rulez Contest - Hon Mention
UBB.classic 6.7.2 - RIP