Hey there!
I want to design a 3d printing part for the dashboard of my golf 6, more precisely the top middle part right 
.
I am somewhat experienced with designing parts in fusion 360 for daily usage, but I don't really know how to design this part.
I did try getting a fit for the front arc and an arc from the middle front to the middle back, then did construct a plane from both arcs and did round the result on both back edges. But I noticed when trying the fit, that the dashboard top is not really symmetric, but has an additional bump to the left.
Do you have any tips/guidance/ideas on how to get a better fit? I did think about scanning it 3d with some lidar device, but I don't have an iPhone and wouldn't know where else to get a device.
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
Hmm I'm not sure whether I did understand correctly. But if it works out that way.. I'll need to take a look at it.
I did slice it to be only the very bottom part of the print to do a test fit. All tests I did before were only slices of the area :)