Cuts both ways. Buy it or make it. I’d say buy the OEM part.
You can grab the Leica IIIc viewfinder plate on the big auction sight for $13 delivered from heavystar.
[re the above feelings, the other week I spent a lot of time trying to remake a broken tripod piece, for a junk Chinese tripod. I liked the tripod (now looking at its website it might be Japanese or Myanmar). Why. I ask that myself after having spent time and stock to only have me test fit it and crack-break a different corresponding piece on the tripod. Some old plastics just don’t hold up well to even a bit of stress. How much time did I spend on this?! I gave up and a used mint Tiltall arrives tomorrow, but $$.]