The minimal thing necessary shouldn't actually be too tricky or expensive to do.
Really, there are two issues with the current emulator available for sale:
1) it includes a display that's too big to fit in a 3.5" drive bay
2) the placement of the SD card slot in the middle of the circuit board means that even if I bought a smaller display myself and put it to one side within a faceplate, the display will block easy access to the SD card slot, or even block access completely.
Consider if an alternative circuit board layout is made that simply moves the SD card to the right-hand-side (and with the SD card slot possibly 'sticking out' a bit, to accomodate the fact that it might be placed behind a display that has a few millimeters of thickness) , but without either display or buttons mounted included & mounted, just the front-panel connector available.
With this arrangement, I could buy a small-form-factor display mysel, and buttons, and mount things in a floppy drive bay without having to make any physical alterations to the case where things are mounted. And I wouldn't end up with a larger display that I had no particular use for, either.
Pre-made front panels could even be sold separately or together with the emulator - with a small-size display and three buttons for a 3.5" drive opening, and with a larger-size front panel with a standard-size display & three buttons that might fit a 5.25" drive opening.
(Alternatively, instead of moving the SD card slot, might it be possible to keep the current circuit board layout, but instead of mounting the SD card slot on the board, connect one via a flat cable or similar and allow it to be positioned separately - similarly to how the front panel can be positioned? Certainly, it seems possible to buy SD card extension cables
so this might be possible.)