The issue with dodgers is that they sometimes shrink over time. So taking the old dodger to use as a pattern, may cause fitment issues with a new one.
Generally a new pattern is used by making one out of clear plastic drop cloth, heavy duty type. That allows for the canvas
maker to mark where the supports and buttons are. Only takes an hour to create a pattern to take back to the shop. There is a lot of work
making a dodger.
I worked at a canvas
shop for a bit.