Ian,I built 3 Wemos clocks as gifts for Christmas, and was intending to make cases out of some walnut. Unfortunately I ran out of time, and decided to try the 3D printed case which you provide the STL for.
I used Hatchbox wood filament to print the case unmodified from your STLs. Online, I read about processing wood filament for staining, and generally followed the recommendations. As you can see from the attached picture, the wood filament is incredibly stringy and leaves lots of strings to clean up - but it was pretty easy (back right).
After removing the supports and strings, I machined the openings the fit each clock. The IN12s vary in size a little all over the map, so custom processing of each case was needed. I used a dremel tool with a small drum sander to adjust the arches where needed, and a knife and small file to do other adjustments. I lightly sanded the case with 220 grit sandpaper, and then gave it a quick rub down with 80 grit sandpaper. The reverse order of the grits helps to create an illusion of grain.
Afterwards, I stained the case with a single coat of mahogany stain followed by one coat of satin clear-coat spray. Note that Gel stain is probably best. I used a really old can of stain, which was pretty thick, and I think that helped the overall look.
I have to say, I am surprised how well this treatment came out.. It approaches the walnut look I was going for, without all the woodworking! Thanks for a great addition to the electronics. - Bill
By the way, you may want to make a couple of small modifications to the STL file, for this or other similar cases.
I had two minor problems with the case back. The first was that the opening for the ESP8266 chip was too small - easily fixed with a little cutting and filing.
The second problem was that the lower part of the back would bow out a bit when installed. To fix that problem, I super-glued a small tab at the bottom of the case (see picture) to catch the installed back and stop the bowing.