Using Luxon, what is the best way to convert a duration into a countdown with the following format T-HH:MM:SS?

Question

I am piping a time value that looks like this PT43H22M15S for example through some array methods so that as it counts it can come out the other side looking like this 43:22:15. The way I do this is like this ...

    return this.millisRemaining$.pipe(
      map((val) => {
        console.log(val);
        const duration = Duration.fromMillis(val);
        console.log(duration);
        const result = duration
          .shiftTo('hours', 'minutes', 'seconds')
          .toString();
        console.log(result);
        return result;
      }),
      map((val) => {
        console.log(`from tap: ${val}`);
        const slicedVal = val.slice(2);
        console.log(slicedVal);
        const replacedVal = slicedVal
          .replace('H', ':')
          .replace('M', ':')
          .replace('S', '');
        console.log(replacedVal);
        return replacedVal;
      })
    );
  }

however, I have an issue. Lets say I want to start with 43 Hours. The output will be 43: and if the preceding number is a 0 it will not show up. like :11 ... :10 ... :9 ...etc How do I get it to display 43:00:00 and :09 ... :08 etc?