Here's a good write-up on Boards > https://techcrunch.com/2016/11/05/what-you-need-to-know-abou...
Ultimately, the profile you're looking for is the Old Warhorse who's worked in and around the same industry/niche you're targeting. They want a piece of the action, willing to invest and provide advice. And importantly, trust you to run the day-to-day. Look for former senior operators/builders with deep personal networks.