I wouldn't use steganography (it's not normally very space-efficient), just dump the payload data into a PNG's comment field. You can do this with exiftool "-Comment<=/path/to/inputfile" dummy.png and re-extract it with exiftool -b -Comment dummy.png > recovered_file. Of course you'd use libpng directly in a FUSE filesystem, but this shows that it's easy enough. Use a 20kB cat picture and 2 MB payload and you achieve 99% space efficiency.

It's probably inconvenient enough for most data hoarders for Amazon not to care. You can still switch to encoding it in the pixel values in case they do. I'm not familiar enough with PNG to comment on the space-efficiency of that.