One of my previous blog posts that is about recreating Atari XL/XE floppy disks describes a SIO2USB interface inside my Atari 65XE. The advantage of it is that it includes a switch that lets you toggle between the USB connection and the SIO port. This way you can easily switch between a real and virtual floppy disk that can be mounted with AspeQt software and copy real floppy disks to disks images and the other way around.
This is how the C64 cartridge cases turned out. They were printed on Creality LD-002R SLA printer. They look ok to me and the two halves fit perfectly well. Just some attention is needed where the supports were attached.