That's where I'm thinking to go...but it's backwards from that I'm used to!
What I've done in the past was start with something blocky but close in form then remove material from the
positive image so what is left is the form you want. Pouring concrete is the opposite...start from a blocky form and remove the
negative image so it can be filled with the concrete to become the
positive image. That's much harder to visualize!
I could cast a mold...that is, build the positive image the way I want and pour silicone or some other process to make the mold to pour the concrete into, but that's a lot of work too!
I might try something like sculptcrete which is also something I've never used
I shouldn't need anything for a few years yet...I think this summer will be some play time to learn/see what's feasible