Once the doublers have dried then the F1 and F2 formers can be added. they shouls be square to both sides. Any suitably right angled object can be used for a square. I've been carting that aluminum block around for 30 years.
The rail between the F1 and F2 is 1/4 x 1/8 hardwood. Along with helping to stiffen the fuselage sides it will be used to mount servos.
