Probably without a 3d scanner, using paper and scissor you will only get a (hopefully) better and better approximation of the real surface.

Anyway, without a 3d scanner, to have some precise measurement you could just use some modeling clay to recreate the correct form and then set some reference point and take measurements from there, it should be precise enough to create a model in Fusion360 that account also for every asymmetry that could be present.