Jim Michalak has a good essay on hollow spars, including calculations for
comparing round and rectangular spars. (Feb'00).
John O'Neill wrote a two part article on birdsmouth spars for Boatbuilder
magazine. The Jul/Aug '01 issue has the calculations for elliptical section
Gregg Carlson has a neat program (available at www.carlsondesign.com) for
calculating strip sizes for round birdsmouths.
Although a birdsmouth enthusiast, I'm curious as to why you would build the
boom this way.