"It's just the hand drawn diagrams that end up in the patent application. I find that trait among inventors amusing."

Isn't the date on which the concept was conceived a critical date?

Design drafts are often started on whiteboards, blackboards, flipcharts or backs of old memos or envelopes. Such a draft, if dated and preferably signed, could serve as evidence of "first to invent".

Coffee or beer stains might add to authenticity.

Just a thought.