Denis, it's valid to treat the"boom" as just another of the battens, as it's doing the same job. If you're happy with your current battens, just get another one.

Having said that, the top sheeted batten is best made a little stronger than the others, as it sees more loading,  and there is no disadvantage to having the boom a little heavier. I suggest you make both of these from aluminium tube of the same diameter as the other battens, but with a thicker wall, or with the same wall thickness, but the next size up in diameter (whatever fits the pockets, if any).