Mine is the much more desirable convertible so it doesn't have a window frame!
However the same principle should work. I cut a strip of garden rubbish bin loader, about 3" wide, I also have a length of plastic "L" shape, about 1cm on each edge.Place the L against the window and then push the thin flexible plastic under it pushing firmly against the window, using duct tape stick the plastic down and the the L to the plastic. I cut a small section out to allow for the front window upright. When all stuck together take the skin off and I placed it upside down on a table leant against a wall (this is all precision work). Now the the plastic is on the table with the weight of the door skin pressing down it forms the correct curve. I placed a piece of timber against the L to stop the door sliding. Then back fill with GRP paste, let it dry and the plastic will peel away, GRP does not stick to it.
If you are not using a door rubber on the sliding window I would pack the L out slightly from the edge of the plastic sheet, that way with a little sanding you should be able to get the gap closed completely. Then its just a matter of sanding down.
When I do the other door I will post some more detailed photos.
The plastic bin loader is very useful. I took a mould of something and used the plastic to make the upright shuts so the mould could be split. I cut it to roughly the contour then placed it upright then ran a felt tip along the panel horizontally to marl the cantor on the shape. Cut it out and it was a very close fit. Held it in place with duct tape and some wooden blocks. When the moulded section had dried I just removed the tape and plastic, put mould release on the newly formed flanged and glassed the next section directly to it. Worked a treat.
Link to bag loader plastic
http://www.ebay.co.uk/itm/Easy-Fill-...oAAOxywXFSb7-B