I had a special square cutter tool specifically designed for this purpose. Just slot the disk in the tool, punch, done.

It helped when you were putting out hundreds of disks of pirated games on a regular basis.

I had a little switch to bypass the optical notch-detector on my 1541 drive.

I still think the single-sided disks were from the same production line as the double sided ones, and it was all market segmentation.

I did the same. And disks were a bit expensive to a kid like me, so I'd even do the cut on various game disks I had, since many of them were single side only, and it was like getting a free disk.

standard paper hole punch worked too, given enough hand strength :)

"... flipped the disk over..."

They called them flippy disks...