This is the case for my Caldera split keyboard. For both the left and right side, there's a top and bottom that get screwed together to create the case.

A video showing the keyboard including the assembly of the case can be found here: 

Assembly instructions for each half:

• Click the key switches into the top plate
• Slide the PCB in and click all the keys into the PCB
• Slide the battery in and connect it to the jack on the PCB (for the battery I used a replacement PS3 controller battery from Amazon for each side)
• Screw the two halves together using 10x M2x3 screws, and 5x M2x4 standoffs for each side (note that for the standoffs, if you instead have heat-set inserts of the same size, you can also just use those, as I made the holes a little wider)
• Optionally add 4x rubber feet (6 mm diameter by 2 mm high) to the bottom of each side

Details on the parts for the actual keyboard's assembly (including PCB, key switches, key caps, etc.) can be found on my blog: <pholder>

Both the top and the bottom should be printed face-down. Everything can technically be printed without supports decently well, but I suggest you use the print profile for Bambu printers that I attached as it paints supports on the relevant areas that need them, but not the ones that don't. If you're doing it manually from the STL, you can either use no supports and it'll probably be fine, or you can paint some normal supports underneath the bottom plate, and for the top plate use some tree supports underneath the battery tabs.

My keyboard in the photos is printed in Bambu marble PLA!