Just my opinion here, some things I could add, because I do hardware mainly, a lot of the time (programming is not my strong point), is that often all the thought on the project gets put into the electronics & software/firmware development and the mechanical engineering, industrial design, artwork etc gets ignored. Projects get released as bare boards etc. I was impressed with the SD2PET, that they had at least crafted a reasonable enclosure for it.
For me something that makes a machine desirable and beautiful, and attractive, it the integration of excellent mechanical and electronics engineering.
Therefore, if I was doing a project like this (though this one is outside my skill set with the firmware) what I would probably do is start with a small extruded aluminium enclosure, such as one made by Takachi, make it up to look like a baby or miniature Commodore dual disk drive unit.
Perhaps even with similar artwork and front panel graphics, with both the SD cards on the front panel for drive 0 and drive 1 analogous to where the disk slots are on the full size Commodore dual drives and even LED's in the same relative places.
Put a standard IEEE-488 connector on the rear panel and another one or two there which allows another device to be connected to the bus, along with a DC jack (to power the unit from a wall wart). Then have a cable with the IEEE-488 connector on one end and the edge connector for the PET on the other (this cable already exists). And the functional goal would be to match the performance and features of the original drives in every respect, but of course much higher storage capacity and much less bench space. And also be a kind of show piece, as a miniature working dual disk drive replica.
Buying one of these units then , would be sort of like buying an original dual drive unit, only everything miniaturized and more reliable, rather than just a bare pcb that hides on the rear of the PET.
A while back I won a design competition for an unbeatable Tic Tac Toe computer. Part of the reason was not so much that my electronic design might have been more clever than any of the other entries, in fact it was somewhat retro with logic gates, but I think because of the industrial design, artwork, presentation, enclosure, ergonomics etc to make an attractive machine. I would always encourage extra work in these areas for an electronic project. Some of the other competition entries were merely masses of wires on proto boards and bare pcb's begging to be shorted out on the nearest conductor: