Creating tool window with toolbar?

I'm trying to use the UI Designer to create a plugin that provides a tool window (similar to the Ant plugin). However, I can't seem to figure out how to get it to create a toolbar similar to the one in other tool windows...

I've got a JPanel on the UI Designer mapped to a class variable (buttonPanel), and I do something like the following:

However, the toolbar in the tool window ends up being too tall, and the button is centered in the middle of the panel. I'll admit I'm a complete newbie to Java layouts, so this is probably entirely my fault...anyone have any pointers?

Thanks!

-- Sean T.