Well you could cheat a little with the built in tools with the control panel.
First goto:
Control Panel » Portal Layout
Select the external use link to the right of the island in question.
When it opens you will see
Code to place inside your PHP enabled webpage.:
Select the whole section starting with <?php up to including ?>
copy it.
Then goto
Control Panel » Display Options
select the HTML Includes tab.
Paste in the
Default header text box.
Click the update button.
It will be centered as default for formatting.
But give it a try.
