I used a LulzBot TAZ 4 with PLA filament.  Layer height was 0.2mm and speed was moderate.  I used 2mm wall thickness and 20% infill.  Because the enclosure is pretty much a shell anyway, the wall thickness and infill can be adjusted, but definitely keep at least 1.5mm wall thickness so the machine screw threads have something to bite into.

Sand the outside of the enclosure as desired to create a more finished, smooth look.  I used up to 2000 grit sandpaper and the results were very nice.  Don't worry about the inside since it won't be visible anyway.

Use a 2.5mm (3/32" if you don't have metric bits) drill bit to prepare the holes in the top enclosure piece for threading.  When I printed the enclosure, the holes were slightly too small to accept the M3x0.5 tap.

Tap the holes in the top enclosure with a M3x0.5 tap.  This should be very easy since the plastic is fairly soft, so you may be able to skip this and directly screw the machine screws into the plastic.

Ensure Components near the screw notches are as far away as possible so there is no  interference with the enclosure.  Use hot glue if necessary to stabilize ENC28J60 module on bottom of PCB.

Using a #2 phillips screw driver, remove the two mounting screws from the stack light.  Fish the wire bundle through the large hole in the top enclosure piece.  Use the two mounting screws to attach the top enclosure to the stack light.

Trim and strip wires so there is enough slack to work with, but it should still fit coiled up in the top enclosure cavity.  About 5-6" should be plenty.  Only strip about 1/4" so it fills the screw terminals, but no extra copper is exposed.

Connect the common wire to the "+" screw terminal, the green light wire to the "G" screw terminal, the yellow light wire to the "Y" screw terminal, and the red light wire to the "R" screw terminal.

Coil wires inside top enclosure cavity and place PCB so it lies flush inside the top enclosure and the PCB screw notches line up with the tapped holes in the top enclosure.

Align bottom enclosure with the top enclosure and insert the four 12mm M3x0.5 screws.  Tighten the screws using a #2 Phillips head screw driver