symbol-observable | Symbol.observable ponyfill | Reactive Programming library

I'm trying to type an object that is observable and uses a custom Symbol.observable ponyfill (similar to the system-observable ponyfill; code shown below), but I can't figure out a way to do it.

I've tried using typeof $$observable for the key and tried making Symbol.observable a unique symbol, but neither of those worked. I've had to resort to casting the object, but the problem with that approach is that I then have to cast the entire object (not shown below) and that makes it harder to catch other type errors due to other properties in the object being incorrectly typed. I also can't just do ReturnType because I'd like to have it defined in a separate interface without having to depend on the function.

I would alternatively be interested in knowing if there's a way to cast it on a more fine-grained level than casting the entire object (if it turns out to be impossible to do without casting).

I've provided a playground link. Any help would be appreciated.

First attempt (no casting)

Error message

Property '[Symbol.observable]' is missing in type '{ [x: string]: () => { [x: string]: ((observer: Observer) => { unsubscribe(): void; }) | (() => { [x: string]: ((observer: Observer) => { unsubscribe(): void; }) | ...; subscribe(observer: Observer): { ...; }; }); subscribe(observer: Observer): { ...; }; }; }' but required in type 'Container'.(2741)

Code