Yea, there's really no way to do an "interview assignment" well.

If you give unlimited amount of time, you're giving an advantage to people with no life who can just focus on your assignment and polish it as if it were a full time job.

If you give a limited amount of time, then you're making the interview a pressure cooker with a countdown clock, giving a disadvantage to people who are just not great at working under minute-to-minute time pressure.