Well after reading through this thread I've come to like two suggestions for the motto.

The first is simply "By Choice." I like this shortened version more than "By Choice, Not Blood" because, to me, that seems redundant. Obviously it's not by blood if it IS by choice. The longer version is sort of like: Completely Green, not Blue.

The second one I like, and I think I like this one more, is "In Kilted Fellowship." Because really, could you not define XMTS as a kilted fellowship?

Haha! Now that's 4 cents from me!