Good job
I use Inkscape for skinning too, but have a bit diffrents sheme: I have a template 1024x1024 pxs with 4 layers there: (from top to bottom) mask -> logos -> colors -> template.
The good point to use layers is when you finished part of work (setting logos for example), you can just lock layer and continue work with next part. You can't move objects on locked layer.
Usually I'm locking 2 layers from begin: template (where I import default jpg car skin) and mask (black mask in svg format which opens only visible parts of skin).
For postprocessing I use Gimp - for adding shadows to skin - just two layers there: default skin with removed default logos ("Multiply" mode and 80% opacity) and skin itself.
