Styrofoam not silkspan
The comments about the problems with silkspan are right on. The solution is regular old styrofoam, the stuff in which products come packed.
Cut it so that it fits the sections between bulkheads....on any model we card types like to build. A bit of rubber cement will hold it in place. Do not use epoxies or glues containing VOC's (volatile organic compound) as they will eat the styrofoam. Elmer's white will hold it in place, too.
Then, using a nichrome wire cutter, trim the styrofoam to shape using the bulkheads as a guide.
Do bit of finish sanding to smooth everything out. With some care, you can even sand in contours so that they appear curved instead of straight lines. This, of course, assumes that you can apply contour to the paper, which you can with careful rolling.
Works like a champ!