Just take pictures. I use FreeCAD. You just add the picture to whatever plane. I usually put a small precision ruler in the image. Then there is a scaling tool. You pick two points in the imported image and it scales the size of the image so that you can easily create sketches that follow the contours.

Don't print big stuff blind. Copy your designed part, then do a couple quick slices so that you only print a tiny unit test to check the fit of critical areas.

Having a set of printed radius gauges helps to measure. GL

Buy or print a contour gauge: https://www.printables.com/model/57209-lockable-contour-gauge

Get the contour you need, trace it onto paper, then get whatever measurements you want, or take a pic and bring it into your modeling program.

I would sit in the car with some pieces of cardboard and scissors and repeatedly fit the edge of a piece of cardboard to the curve horizontally at the lowest relevant height, then repeat the process at a fixed interval higher up. You can then measure the cardboard pieces to form a mesh.

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.

This is gonna sound really dumb, but get something to make a negative, like a putty or foam or something. Make a negative of the contour, take the negative and trace it on a piece of paper. Scan the paper with a fax machine or scanner from a printer. Export that as an SVG, import it into fusion. Messure you negatives size and scale the image to the negative. Trace the line of the image in fusion.

This will get you within throwing range of fitting.

Get some childrens playdough, use something rigid like a 3d printed flat box to get the shape negative. Now you can use silicone or sth similar to get the exact shape and attach that part to the piece you actually want to attach. Perfekt shape even for weird geometries, also you can simply change the part that actually needs to be there with out any issues.