As MoR suggested I went with the first Scottish name on any branch of my tree. In my case this is on my mother's side. I also considered the state of my birth (no...not neckid) Idaho, Ulster for my Irish ancestors, and German-American.

I am not much of a joiner but I did join the Clan Morrison Society of North America just to make things official like and have met some nice folks.

I highly recommend ordering swatches of tartan so you get a good idea what the kilt will look like.