A reed mat can easily be cut to size. Shortening a reed mat in height can be done with clippers or hedge shears. Cut the reed to the desired height, and place the reed mat in the desired spot.
It is also easy to adjust the width. Choose the right size and add 3 reeds to it. Cut the iron wires from top to bottom and remove the last three stems. This leaves a few centimeters of iron wire which can be easily twisted together again so that the reed mat stays firmly together.