Change the margin values to shift the close button to the required position.
By default WebPlus puts the close button above right of the top of the lightbox.
Click here to see the result of the code on the left.
Create a quickshape that opens your lightbox on your main page, the same colour as the background and with no border so that it is invisible.
Give it an ID of say 'popitup'.
Create a button on your page that appears in your iFrame with an onclick event like :
Simple as that! Demo in iFrame on the right.