You could also ask for royalties with an “up front minimum” or “yearly minimum”. This gives you lots of headroom but also gives you something to plan on. Stock can be tricky as there are a lot of factors that go into valuation of a company’s worth and stock price. My advice would be to talk to a financial consultant and a lawyer. Collaborations are good until they aren’t. It is good to have a document that spells out the terms in a way both party is clear about.