Yes, copy left is really your best bet. While 'technically' not open source, there is also parity ( https://licensezero.com/licenses/parity https://licensezero.

Yes, copy left is really your best bet. While 'technically' not open source, there is also parity ( https://licensezero.com/licenses/parity https://licensezero.com/licenses/parity ) which I think brings a few interesting ideas to this space. All of those options make It harder for your open source users though.