You’re gonna get people telling you you need a footer under this, which you do. And you’ll need to mortar them together, those are not landscaping blocks. That said, I would just build them using half bond, and not glue anything if you’re not going to go the whole nine yards and do it right. It will sink. It will fall over at some point. Taking this as a give-in, make it as re-usable as possible and make it so you can just fix it easily when it fails. Because it will fail. This is not a knock on you. It would fail if I built it, or anyone looking at built it.