The genius has figured it out. All around the interior of the wagon, there are vertical seams between one piece of the vinyl interior and the next, into which I can slip something the thickness of a dinner knife blade. I've got a roll of plumber's tape, that galvanized stuff 3/4 inch wide, which can be cut with tin snips, easily bent, and filed. I'll cut a piece about 3" long, bend it into a Z-shape, insert one end, leaving the other V-shaped end outside, over which a loop of the fishing line will go. The seams are conveniently located for stringing the fishing line across the windows, doors, back hatch, and across the width of the car behind the driver's seat.
The curtain material will be hemmed for a curtain rod top and bottom. The fishing line "curtain rod" will be threaded thru the top edge, and disk magnets sewn into the bottom edge like you see on some shower curtains. The magnets stick to the steel roof through the cloth headliner, and they also stick to all the interior metal trim, which is conveniently located below the windows. When you want to close the curtains, you just attach the magnets to the window trim. When you want to open them you attach them to the ceiling. The result should be very convenient, the curtains will be completely out of the way when I'm not using them, and they're easily removable for laundering. When I get this done, I'll take some pix and post them. The only thing I havent figured out is this: the plumber's tape is easy to file into the shape I want and to take off all the rough edges, but I'd like to make the hook part especially smooth. I'm looking for a glue, or silicone, or lacquer or something that I can dip the hook in, which will be kind of thick and will pool enough and dry fast enough to form a smooth coating. Any suggestions?