With the top sheeting on part of the box it becomes more rigid so the sides can be pulled in at the nose & formers fitted.

When I build fuselages I put marks on the middle of the formers with a pencil & draw a line down the centre of the building board. If the formers will not be sitting on the board I extend the centre marks down with little balsa tabs like you can see on the front former. When the sides are joined I tape them top & bottom to hold them together while the glue dries, line the lines on the formers up with the centre line on the board & hold them there with bricks. No more banana shaped fuselages!

As this model relys on the wing tube for alignment I block the tube up every time I am working on the fuselage to ensure it all stays square.