The plan is to use .016 2024 T3 (stated in my post above) for the skin which will be stressed skin like the Zenith 701 uses in the rear fuse although the angles they use at the corners are heavier.In post 120, you seemed to show diagonals on your "keel", here, not so much. Without them, or a skin of some sort, your torsional strength and stiffness will be poor. And I know from personal experience that getting long bends in thin aluminum with the tooling available to amateurs is TOUGH, and in a beam like this, if they are not straight, they will fail in compression (buckling) quite readily.
New Kolb sells boom tubes for $500 plus shipping, just sayin'
The fuselage of a conventional light plane is loaded in fight with the bottom of the fuselage is in compression, yet your keel beam has more material on top, where the loads, especially on a tri-gear airplane (no landing loads on the tail in a NORMAL landing), are in tension.
Look at how spars are built, particularly box spars, for some ideas on how to improve this.