Use the leading edge of the wing as a fixed reference point...and model outward from there.

For example, when adding the booms position them from where they meet the leading edge. That will put the booms in the right spot and make them the right width.

Other features can be added by using each new known location as a reference for a new piece.

You will still build in some inaccuracies...but they will be slight enough to not matter all that much.