I have contemplated and sketched out what would be required to build a slot loading sd card reader. Ultimately, I believe a vertical mount door loading mechanism* is cheaper, more reliable, and certainly has a decent aesthetic.

* much like a vertically mounted cd player

Did anyone ever use them for data storage on computers? IIRC they held about 200MB so they'd have been a welcome (and robust) upgrade from Zip drives.

I know there were "different" disks for the Yamaha multitrackers, but I'm not sure what was different about them.