If he is using a monsoon it has no attachment methods other than suction cups. Your thinking of corner wedges you buy from mistking/aquazamp, not the monsoon.
You can user thin plastic wrapped bread ties like they put on power cords when you buy a piece of electronics, put the end poking out of the screen then put your nozzle thru the loop and tie it on the outside..
Cut a 3" x 3" square of it . cut 4 holes in each corner. Then place on the inside of the cage and use small zipties to hold it in place at the 4 corners. Then use the suction cup to suction to the center of the plactic 3" square as you would on a glass enclosure. Then adjust the nozzle to spray where you like.