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.
Sony did release drives, DGR looks at one in this video for example https://www.youtube.com/watch?v=CzwtCBj5jEs . It's yet another example in the weird graveyard of replacements for the 1.44MB floppy that didn't catch on.
* much like a vertically mounted cd player