I made a spacer from hardboard sized to fit the saw opening. It has to be just thick enough to clear the blade’s teeth when the insert blank is set in the opening. Cut an oversize slot for the blade with a jig saw and it’s ready to use.
To cut the blade slot in a new zero-clearance insert blank, I lower the blade and place the spacer in the opening. Put a blank insert on top and clamp it down. Turn on the saw and slowly raise the blade until it cuts through the blank.