Floating DIV tag JS code that users can drop a snipet of code into their site

Hi All
Not sure how to do this or if it is possible. I want to give my customers a small piece of code that they can drop into the head section of any page
that will give them a floating div tag to say bookonline etc.

I know it can be done with CSS but as this will be added to lots of different website I wanted something maybe a remote JS file on our server that could be pasted into the head
section of our website without having to add CSS and div tags elements to their pages.

Any ideas you may have seen before

Cheers
Steve