What's the best way to define a module-local struct?

There is no way to do that. Structs are a module based definition for specialized maps and just like the defining modules themselfs are globally accessible so are their struct features.

The best way I know, which you may well also know, is the submodule syntax with @moduledoc false:

defmodule Foo do
  defmodule Bar do
    @moduledoc false

    defstruct [:one, :two, :three]
  end
end

Elixir doesn’t have private modules so @moduledoc false is the next best thing (I actually don’t mind this).

Note: It is not available at compile time (at least not that I know of), ie, you can’t use it in the module body. Inside of functions is fine.

Both answers so far address limiting external accessibility, but in my literal reading of the OP I don’t see that concern. So my naive answer is: just do a defstruct in the module which will be the sole consumer of the struct.

But I must be reading it wrong.

Yeah that’s an option, but if that’s the only one I’d rather keep it a Map, as my hunch says that no description might be better than a misleading description.