I don’t mean to be discouraging, but in my experience this is a total and complete crapshoot. I would put the disclaimer on the actual file, but there’s no guarantee everyone will take it to heart. Something short and sweet, and easy to understand. Maybe something like “not for commercial use, even if altered.” I’m not really good at phrasing stuff, but you want something that doesn’t require further research, because people just won’t do it.
And guaranteed, you’re going to find your design being used in a way you don’t want. It’s inevitable. There are two ways to handle it. First, rant angrily at your screen every time you find your design on etsy being used illegally and then move on. Second, familiarize yourself with DMCA and how the whole process works. Loads of people hate on it, I know, but it really makes copyright claims quick and easy from the perspective of the copyright holder (that’s you), and allows you to protect more of your stuff than you would otherwise be able to. But, I mean, note that I didn’t say ‘all of your stuff.’ And when I say quick and easy, it’s completely relative. It’s quick and easy to file a take down notice against one person. Doing it against 100 people is a part time job. Which is where the rant and move on option comes in.
The other thing worth noting is that attaching a disclaimer that someone violates (like, by altering your design and then selling it) doesn’t automatically make you right. It makes you more likely to be right, but keep in mind that copyright violations are determined in court. Meaning you, your lawyer, and anyone else except the judge in your court case can’t really say with 100% certainty who is right. Plenty of people have lost what seem like straight forward cases.
Lastly, decide now how much you care about this. If you’re just trying to stop most people from using your design for profit, you should be fine. Most people are awesome, and won’t sell your design if you don’t want them to. If you really feel very strong about it, and seeing it used commercially would really be devastating, you’re probably better off not putting it out there.