The correct phrase is: "Hope you had a stress-free day."
The phrase "Hope you had a stress-free day" is in the past tense and is commonly used to express a wish for someone to have experienced a day without stress. It implies that the day has already passed, and you are expressing your hope that it went well for the person.
On the other hand, "hope you have had a stressed-free day" seems to be mixing verb tenses incorrectly. It combines the present tense "hope you have" with the past tense "had a stressed-free day," which does not flow naturally.
So, the correct and natural way to express the sentiment is by using the past tense: "Hope you had a stress-free day."