Here are some pictures of the modifications to the case to make it fit the “C”:
First image: This is needed if you intend to use an eMMC module. The CPU hump is cut down only in the region where it intersects with the eMMC module. Note that this cut does NOT touch any part where the CPU actually contacts the hump. This cut could be made smaller, if you have a CNC machine available. I did it with a air powered die grinder with an abrasive disk.
Second image: The hole for the power jack is not aligned to the actual power jack. A small rat tail file is perfect to adjust this and only takes a few seconds.
Third image: The holes for the USB-A ports need to be expanded, and moved. On the “A/B” boards, the USB sockets don’t extend as far as on the “C” board. Where on the A/B the USB socket butts up to the cover plate, on the C board, it has to actually slide into the plate. This means that the hole in the plate needs to be filed larger in order to fit the socket inside it. In addition, the socket closest to the side of the board is moved over slightly, around 1/8 of an inch, so the hole for that one needs to be moved over slightly.
The final consideration that must be made for fitting the “C” board into this case, is that when it is assembled, there is a small gap between the CPU and the CPU hump. By eye, it looks like its around 1/2 mm. Since the A/B and the C boards all have exactly the same CPU, the only explanation I can see for this is that since the board is mounted from the top, the PCB must be a little thicker on the A/B than on the C. This gap has to be filled with a good conductive material, and I don’t think that thermal paste is really the appropriate choice. That stuff isn’t made for filling voids. But thermal PADS are made for filling voids. I used this, 1/2 mm Arctic: https://www.amazon.ca/Arctic-FSU010110-Efficient-Conductivity-Handling/dp/B00UYTT3I2 (I actually bought a much bigger sheet of the same stuff, because I know that I’ll eventually use it)
Note that these adjustments can be very easily incorporated into the design of the case. The changes would NOT break compatibility with the A/B boards.
Further changes that I would recommend to the design of this case;
The CPU bump should be raised by 0.5mm, and instead of screwing the board to the plexiglass cover, leave 4 bumps in the base that align with the mounting holes of the SBC, and drill and tap those. This would make it so that both the A/B and C board would mount into it without needing to fill the gap with a thermal pad, since they would both mount on the CPU side. Note: it is necessary to raise the CPU bump, because there is an inductor on the C board that would contact the case before the CPU makes contact.
Additional advantages to this change are that it would make the mounting more robust, and that it would make the case compatible with some mezzanine boards since it could be used with the plexiglass cover removed. With the current design, attempting to use it with the cover removed would be dangerous, since there won’t be anything to keep the board standing straight – it could fall over and short out.