The first few pixels of the leading edge of the wing top and bottom are stretched IIRC. Over this portion its important the lines on the actual bmp are purely horizontal or they will look smudged on the skin.
The outer rim of the vent is a rounded rectangle skewed a few degrees. Half of this box was moved to the upper and half to the lower wing panel. The vent itself is the same thing but smaller and filled with black. I would have used the viewer and trial and error to get the position of the upper and lower halves matched.
Couldn't you just paste one of my default skins into yours as a layer and use it to get the vent position and shape right?