Do you have instructions? I can't tell from your pictures what you did. Is the stem glued to the flat top, or is there a notch in the top of the cap?
Oh, holy crap.. I haven't been on here in ages, I'm so sorry.
If it is still relevant: I simple cut a small piece of wood 4mm thick, 3mm wide, 4mm long (I think it was), with the cross for mx stems in the middle. I then glued that to a piece of acrylic that was cut in the approximate shape of a "normal" keycap.