Source: Powell A. Casey, "Encylcopedia of Forts, Posts, Named Camps, and Other Military Installations in Louisiana, 1700-1981" (Claitor's Publishing, Baton Rouge, 1983).
Casey provides a brief description of Fort Beauregard at Harrisonburg on pages 23-24 and cites his sources as follows:
(1) Edwin C. Bearss, "Story of Fort Beauregard" in Louisiana Studies, Volume III, 330-384, and Volume IV, 3-40. He doesn't tell us who the publisher is (the name of the Louisiana Historical Association's quarterly publication is "Louisiana History"), but someone at the State Library of Louisiana in Baton Rouge should be able to help you find copies of this. You should be able to obtain a copy through Interlibrary Loan. http://www.state.lib.la.us/Dept/LaSect/index.htm
(2) Official Records, Volume 26, Part I, pp 273-283. These are online in a searchable database at http://library5.library.cornell.edu/moa/moa_browse.html
I would bet Ed Bearss got most of the "whole" story!