Good question, I wonder the same. I would say E-ink is still the best bet in terms of no-light emission property. Someone from ReMarkable could give really good insights as they are the hottest startup building a product within this space. Other than that possibly the cheapest option would be to tile old kindles and find a way to interface with them...

Sad that Amazon makes Kindle very closed to modification in terms of software. Therefore I am a huge fan of ReMarkable because an underdog may allow us to finally build e-ink apps: https://github.com/reHackable/awesome-reMarkable