Ask HN: Employer 'Outside work' agreements

Hi there - we have a lot of developers in our company and we would like to come up with a reasonable employee 'outside work' agreement. We're open to devs moonlighting on their own stuff and getting paid for their own stuff, but not on company time.

Is there a good example agreement out there that is reasonable for both the employer and the employee? We'd love for employees that invent things OUTSIDE of work time and they should OWN them.