Flow Hood. Hold it over the opening and read the scale. Problem solved.
Flow hood is the best.... cover and read, cover and read. What was the old fashion way? traverse with a tip that would pick up velocity and somehow it would cancel static to get only velocity and then you get the average of the sum. A few more steps and you have CFM.
I would like to try a flow hood against an anemometer or a u tube to see how close the readings would be.
A traverse is best and most accurate. But just to get a ball park, you can measure velocity in a couple of places across the duct, take the average, and multiply by the square footage of the duct. Hot wire anemometers are pretty cheap.