Edit: I built a second one for a colleague
Seeing as the deej was mentioned with Linux-compatibility, I decided to build one with the LTT-Case.
Hardware
As Jordan mentioned above, the hole for the USB-C cable can be undersized depending on your Cable. I just sanded it until the Cable I wanted to use fit.
For the screws I used some m3x10 self tappers for the smaller screws and m3.5x25 self tappers for the longer ones. In hindsight the longer screws should be m3x20 self tappers, otherwise they might come out the other side.
With the microcontroller you really want to solder the contacts to the pcb (or pins if they are already attached). Otherwise your lid will not close.
Linux
The Linux compatibility works, but requires some additional set-up. It also only uses PulseAudio, meaning you cannot adjust the volume of Pipewire applications (you can but only with the master-volume). The only occasion where this was a problem for me has been Minecraft.
Additional setup:
flashing the SparkFun Pro Micro
the pins you want to use are A0, A1, A2, A3 and A6
all required information is included in the docs
https://learn.sparkfun.com/tutorials/qwiic-pro-micro-usb-c-atmega32u4-hookup-guide
using the correct serial port
You need to define a udev rule so that your microcontroller is always available under the same path. Otherwise the path will change as soon as you unplug/replug or reboot.
get the vendorid and productid of your microcontroller with lsusb
add a udev rule as described here https://unix.stackexchange.com/questions/66901/how-to-bind-usb-device-under-a-static-name
set the com_port field in the deej config to /dev/my_uart (or whatever you set)