I made a coroplast & aluminum flashing undertray similar to basjoos:
Looking aft:
Looking forward:
(Note, everything aft of the muffler is part of the bottom of the prototype boat tail.)
It's been on & off the car multiple times: for testing; to apply anti-rust treatment to the car; and because I won't risk crossing the Canada/US border with it on the vehicle. (I just assume it'll get me flagged for further unnecessary inspection.)
Mine is directly attached to the floorpan/underbody and other existing hard points with machine screws.
You can't see it well in the photo, but the front section is cut out where the exhaust down tube goes horizontal into the catalytic converter, and also a bit aft of it. There's about a 1 inch margin around the metal. Aside from that, it completely covers the engine bay.
There are cutouts to permit full lock steering of the front tires.
The control arms of the independent rear suspension are fully covered, but they still have full range of motion because the coroplast is layered at that point (think shingles) so that as the suspension moves the control arm up & down, the layers slide.
Results: I measured a 2.2% improvement in fuel economy @ 90 km/h (56 mph) in an A-B experiment. See
http://ecomodder.com/forum/showthrea...9.html#post873