A couple of poor quality pictures:
Above: from the side looking back.
Same as above.
From rear looking forward: you can see the cutouts for the rear strut perches/mounts and how the coroplast tucks into the rearmost section.
Overview:
Notes:
Blue outline is the main section of undertray with additional
Green sections at the rear and out sides.
1) A full width (~48 inch) coroplast sheet fits perfectly between the rear wheels (with cutouts for the rear strut mounts/perches)
2) Red
"X" indicates attachment point - screws into factory plastic undertrays, except "A" and "C".
3)
"A" is a crossmember in the exhaust tunnel. I fished some wire through a hole in it, and twist tied it to the coro.
4)
"B" is a cutout I made for the exhaust pipe, which would have touched the coroplast there.
5)
"C" because the twist beam rear axle has to move, you can't make a fixed attachment to it, or it'll pull against the other screws. So I used elastic cord (bungee cord) wrapped twice around the axle+coro to secure it there.
6) The main (blue) sheet tucks in (above) a smaller fixed rear sheet, and it's free to "float" above it to allow some movement caused by movement at the rear axle.
7) You'll have to come up with your own way to make cutouts in the rearmost undertray for the muffler. There was no muffler on the car when I was doing all this -- It fell off months ago.