I have written about immigration in a number of places, including a section of Chapter 10 of my Libertarian Philosophy in the Real World, and in a number of blog posts. In light of what I have learned in my many subsequent discussions/debates of this subject, I am in the process of consolidating my thoughts in a systematic way for publication in a philosophy journal. Thus, while I have some half-baked ideas on other topics that I may eventually share, I am taking a few weeks off to finish this paper.
UPDATE (5/24/18): I have been fortunate in getting my friend, Danny Frederick, to co-author this paper with me, thereby exponentially improving it. We have finished our draft, and have begin the process of journal submission. Stay tuned for further developments.