I don't know how you make txz instead of tgz. But you can't have the destination directory inside the directory you are making a package of. So there, make the package to some other directory. You forget file extension as well, so I suggest something like:
You could add more information, like build architecture and such if you like, but you can name it what you want to. Perhaps you need to gunzip the archive, and then compress it again the right file extension to make it txz. I've never done this, but it should work, I suppose.
Ok, thanks to el_b for pointing out that you can actually use .txz directly in makepkg.