Like many others, I've somehow managed to lose a keycap from my K400r. The first and last bottom row keys have different stem geometry compared to the other keys I've checked, it's rectangular instead of square and has an upper guide ridge on the stem instead of in the slot on the base (see second photo). I've uploaded both the embossed "Ctrl" and blank face versions, the blank can also be printed mirrored on X axis to replace the right arrow/end key although it will end up slightly low and to the right (I based the first iteration on the right arrow/end key and had to tweak stem location to work for Left Ctrl).

I'm using Anycubic Photon Workshop for resin printing and had the best luck with 25 degree rotation on the X axis and medium supports, the important part is to make sure no support are on the outer sides of the stem as it can cause binding issues and require delicate cleanup (see third pic for slicer preview). As I never had luck with printing keycaps on an FDM printer and don't have anything finer than a 0.4mm nozzle, no attempt to make it easier to print via FDM was made.