I suspect you already know this, but a reminder for the newer guys reading this - "suction"/"vacuum" doesnt get the dirt up the hose, technically it is air molecules
pushing the dirt up. Yes the suction/vacuum is what makes the air flow so I realize I'm being a tech weenie, but here's the point: if you don't have a way for air to get into the hose, it doesn't lift the dirt. Hence the need for notches or holes, or for the customer to keep the nozzle at an angle or a little bit off the surface.
So a more direct answer: I usually cut 2 notches with a strong pair of wire clippers. I put them closer to the ends/outside of the nozzle, figuring that helps lift the dirt around the edges & corners of the car floor. Then I drag the nozzle across the concrete floor a couple times to round off the sharp edges. Yeah a file would be more professional
iirc I started doing this after that long-ago thread Greg mentioned (decades??), perhaps after IB was bragging about his notching tool?