Ok here is the script i'm using:
[ div="float: left; position:relative; height: 400px; width: 430px; margin: 0px; padding: 10px; background-color: grey; opacity: 0.8; overflow: auto;" ] [/div]
Legend:
Red is the position on the page e.g. left, right or center
Blue is the background color
Green is what makes the scroll bar
Purple is what you use to close the box, same as a spoiler