RTP Dialog Box - stay centered

I have a box that pops up after inactivity -- but I really want some action if they proceed, not just moving on, so i'd like the box to stay on the screen until they close it or click it. I added style="position:fixed" and it kept my image on the screen, but the "x" to close the popup didn't stay with it. It looks like that's inserted a level above what I have access to control -- does anyone know if this is possible to achieve? THANKS!