"Does anyone know a source for the JKF case only?"
I do not know where to find JKF cases now, but I have one from a few years back and it is pretty good (accepts oem spec crystal etc.) except for one thing:
On mine, the spring bar holes are too close to the tops of the lugs, and one is almost into the bevel (all the bevels are fairly shallow). Many cases are made this way, not just JKF.
When looking at a case for sale, I always check this because if the lug holes are to be bored out to 1.25mm or 1.30mm for oem spec spring bars, you can get in a mess if the holes are too high or low to begin with.
Back in time...
Otoh, all five of my MBK cases have holes in the correct location and look fine after boring the holes out for oem spec spring bars.
About 15 years ago, I bought about ten DW (David Wong) 5512/5513/1680 cases (low prices back then compared to now) and only one case had one spring bar hole too high. Not too bad considering the price.
Before I bought the DW cases, I bought some 5512/5513/1680 watches from an internet guy called 'Paul' (Abay.com back then) and the cases had oem spec case necks (accepted oem crystals) and oem spec case tube threads with properly located lug holes. The cases were a little bit out of spec because they had 'spring wire' bezels, all were made for 26.5mm dials, and the case back gasket groove was cut in the case instead of in the case back, but it did not show. Still have a '5514' from back then and they came with oem spec helium valves.
The lug hole placement defect is small to many and easily ignored, but it bothers me because it takes away from the overall appearance of a watch.
Btw...this same problem is present in many cases used for '1016' projects and they have thinner lugs making it harder to bore the holes properly.
Lug holes too high or low makes them look 'over polished'.
All imho.