The main problem with the strength in the tail is that there are so few longerons. The problem of structural strength was solved by the internal truss. However, the covering is only going to be supported by the longerons, so they have to be plenty strong. There is no point in a strong aircraft, where a finger can bend the longerons and distort the fabric covering.

I did some experiments with bending sticks of spruce this afternoon. It is quite clear that the longerons must be at least 6 mm thich, to prevent any bending. Anything less than 6 mm was too weak, and anything over was difficult to shape. If 6 mm does not prove to be strong enough on the real aircraft, I will add some balsa supports from the truss to the longerons, to make them more rigid.

The actual aircraft uses 1/2″ pipes, which translates to approx. 2.5mm on the model. I have decided to round that up to 3 mm. The dimensions for a longeron will then be 3 x 6 mm and arranged as in the picture.