The housing needs to be 3D printed. The bottom piece does not need supports. The top piece should have supports for the holes where the pegs in the bottom piece slide in. Play with the support settings in the slicer (I used Cura), so that you have you have enough room on the X/Y axis to remove them from the holes. I used 20% infill and three perimeter walls. Your mileage may vary. You may want to use horizontal expansion for the holes on the top piece so the pegs on the bottom slide in.

The pegs in the bottom piece should slide into the holes on the top piece. If they don't, take a drill bit and widen the holes a bit. The pegs should be snug. I also used glue to keep the two parts together, though this may not be necessary.

To make the housing appear smooth, I used wood filler to fill the seam between the top and bottom pieces. I then sanded the pieces to smooth them.

After sanding, I primed the housing with automotive primer-filler and re-sanded the housing. Finally, I painted it.

This step is fairly explanatory. The only difficult part was soldering the WS2812B-2020 LEDs. For that step, I used solder paste and a hot air gun. An oven would have been easier, but I didn't have one. The through hole components were soldered with flux and normal leaded solder.

For this step, you can use a soldering iron or a specilized tool. I used my soldering iron to heat and press down on the inserts to melt them into place. But some brass insert sets come with a special tool to use with a soldering iron. In either event, you'll want the top of the inserts to be flush with the housing.

Use M3 screws to attach the board to the housing.

I used Makecode blocks as a proof of concept. But presumably any language that can control a Micro:bit, the RTC, and the LEDs could be used. For the Makecode route, you'll want to set the current time in the "on start" block, and then load the code on the Micro:bit. This will load the time in the RTC, which will save the time if the clock is unplugged. Then, remove the date/time setting blocks and re-upload the code. If you do not do that, the date and time will be reset whenever the clock is turned on.

The clock should now run when plugged in and the switch is turned on.