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