Ask HN: How should I structure a simple co-founder agreement?

I recently brought on a co-founder. I offered a salary and 15% equity with a one year cliff and a four year vest. I have not raised outside funds.

I'm thinking I write a simple document stating 15% equity with a one year cliff and a four year vest and we both sign it and hold a copy.

I'm thinking this will work to cover us both legally until the time of a funding event when we can have lawyers draw up something more official.

Will the simple letter suffice or should I go the lawyer route now?