I think I have my plan after wandering around for an hour in Home Depot. I found a plastic lattice frame that usually goes around a plastic lattice. It's shaped like a U, about an inch tall and perhaps 3/4 inch wide, with about 1/4 inch gap. It comes in 8' length. Then I found a corrugated PVC panels, coming in 8' and 12' lengths.
This is what I am going to do. I'm going to screw the frame directly to the deck board, the gap facing down, right next to the joist, and repeat it again on the other side. Then I'm going to slide each edge of the PVC panel into the gaps. Since the gap is much too wide to hold the panel edge securely, I'll just cut a length of thin plyboard to jimmy into the gap and hold the edge in tightly. A liberal dose of glue (whatever kind that'll adhere to PVC and/or wood) wouldn't hurt. Since the PVC panels come in 36" width and the space between the joists is 16" I'll probably have to cut to reduce the width to 20" or so. That should give me enough of "valley" so the water will channel.
The lattice frame is around $4 each at 8' ft length and I need at least 30 (12' x 16' deck) and the panels are about $12 each but they come in 12' so I need only 12. Add misc. stuff plus gutter and I figure I can do this under $350.
I also found that roll of aluminum flashing can substitute for the PVC, provided that I bend it into a stylized V shape. (bend at the middle and at the both edges). I figure I can roll it out to right length and set it atop a foam board, measure to the center and lay a dimension lumber to serve as guide, and roll down a pizza cutter to make the "groove." Of course you can use other tools but I'm just using what I have available. :)