Get some of the presentation boards used for stuff like science fair projects. Those are very sturdy and stiff cardboard. Also get some glue sticks. Print the map at your desired scale, trim off the margins, and glue it to the board. Either trim the board (if you're okay using it once) to size or don't (if you want to later glue another map to it).


Anything else would be far more involved and incur significantly more expense.