Hi Paul,

Do you have an image? It would be useful to view it. If you did, you might consider cropping the item out of the surrounding area and then saving it to a photo editing file, such as Picasa. You could then manipulate the image to the correct scale size, print the image, cut around its edge right on the line and then set atop Tamiya tape (working against non-stick side). Cut the image perimeter again to make the shape on the tape surface, and remove the inner piece, leaving just the oddly shaped 'hole'. Trim the tape to clean, straight outside edges and set in place on the model where it's required and spray the 'hole'. Masks always tend to work at opposites or reversals of the originals, in this case, by removing the object centre and using its edge to recreate the centre again on a different surface, and with a different medium (tape).

Alternatively as its a panel you could measure the key points on the kit drawing and recreate them on the model in a sort of 'join the dots' procedure.

Yet again, if you have a drawing, use tracing paper to recreate the image and transfer it onto tape, using the opposites rule to get the shape. The rule being that anything within areas defined by a tape edge is subject to paint.

Naturally, if I could see the shape the advice may be simpler after all.