I'm not really sure, but you could just make what you said - a wall count down calendar where you stack a whole bunch of little square papers and staple them onto some heavier poster paper and decorate the poster paper.
Or you could use a little chalk board and just write the count down days on that.
Or what I'm doing is I bought some of the Disney advent calendars for cheap after Christmas and am using those a count down calendars, counting backwards from 25 to 1.
You could even make an advent type calendar yourself (with pictures, not chocolate), where you open a door every day counting down to day 1 being the day you leave...