The reason for use of “expensive cardboard” is two fold. Quality conservation materials that are acid free are more expensive. The other reason for the cost is chemcals and process for the color of the mat board.
As far as the method of achieving the window, for anyone setup for production mat cutting it will be much more labor intensive to assemble the four pieces than to cut the window. Especially since you have to cut the 4 pieces you are going to assemble before you start taking time to assemble them.
If you are concerned about conservation then you also to have to worry about making sure the adhesive used to assemble your mat will not cause the image to degrade.
Originally Posted by dancqu