I think your right about the possible contamination. I had thought about cutting the keg and just using the corny top like you said, but was wondering how well it would seal up, might have to flatten the out a bit where the lid will go?
Nice! Did you make the plastic piece for the top? any drawings for that? specs? :) i have a hobby cnc machine and could probably make one, would take me a bit to design one trial and error.