I'm wondering whether there is a SAFE-like document, except the value would be correlated to hours worked and hourly rate.

The situation I often find myself in is that I am asked to help out startups but they don't have funding yet. I'm happy to help in some cases, but I want to be compensated in equity in a fair way. It seems as though a convertible note would work well - except the value would need to change based on the hours I contribute.