(And also, to give you a piece of code, you would probably have to tell us how the coordinates are saved in your variables.)
This would be one... but... probably as simple snippet as this would not be what you want?
However, I would like to recommend to dump info to the CIW rather than trying some fancy popup window. Secondly, another nice feature is to use the ruler tool to display some certain areas. You could for example write a snippet that creates four indicators of your box, or even draws the box using the ruler layer (no need to open the layout view in write mode).
;; xLL and yLL have to be defined
(setq LL (list xLL yLL))
;; xUR and yUR have to be defined
(setq UR (list xUR yUR))
(printf "::> LL: %L -- UR: %L\n" LL UR)
or
(defun printCoords (LL UR)
(printf "LL: %L :: UR: %L \n" LL UR)
)
and then tie printCoords to one of your bindkeys, where you cleverly have to direct your input parameters LL and UR to the function.