I know Phantom, I completely understand, I've tried doing something like this myself and it's not easy, it's quite time consuming and all, but when it comes to law there are copyright rules. There is nothing you can do actualy, you can't copyright your work, jst b/c those models are already copyrighted no matter the work you've put in.
I do understand what you mean tho, modding is not that easy. But like I said, by law, there is nothing you can do other than encrypt your files or something like that.