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.