Here’s a homebrew kamigoma, using the lip of a soup can, cut to measurements and buffed with emery cloth to smooth the edges. The thickness of the kamigoma under the strings is a little more than 1mm, because the metal is rolled there.
Here’s a photo on imgur:
Per Kyle’s measurements, it’s about 16mm long, 8mm deep, bent so there’s 2-3mm lip under the strings, and 5mm extending down into the pegbox slot. The plate can be glued – I used contact cement, so the plate can be pried off later if necessary.