For those in interested in the Powers theory of modality and argument for a necessary being it generates I would heartily suggest you check out some of Chad Vance works.

A short sketch of such an account can be found in his article:

Dispositional Modal Truthmakers and the Necessary Origin

For a full length work on the same topic (it makes a nice accompaniment to Pruss book on the subject as it goes into necessity of origin issues in some depth) check out his thesis:

In Defense of the New Actualism: Dispositional Modal Truthmakers and the Branching Conception of Possibility"