-- Last Updated: Mar-20-13 7:36 PM EST --
Well, g2d knows way more about attaching tie-downs and other modifications than I do, and I'd never question his choice of materials or adhesives or working methods. However, the ways in which sheet-like materials can be made more rigid by means of adjusting their shape or adding "beams" of specific shape is not a matter of speculation. The basic principles are pretty simple. It only gets complex when they are designed for a particular type and magnitude of load. From what you say, I don't imagine you "see" these things when you look at the world, but examples are everywhere.
Manufacturer's won't or can't always tell you what you want to know about additions or modifications. Here's a perfect example: I had a 1980 Subaru years ago, and Subaru specifically told me that under NO circumstances should a trailer hitch be installed, because the body of the car "wasn't suitable" for hitch installation and because the car "wasn't capable" of pulling a trailer. We built a trailer hitch anyway, from leftover scraps of steel that were at hand, and designed it to attach at three of the best-reinforced locations on the car body, with the hitch frame between those locations being strong enough to insure that any stress was spread between the attachment points, all using the same principles I applied in my post above. Not only did the car pull trailers weighing 700 or 800 pounds with ease (it turns out that Subaru's official statement about this was dead wrong!), but the hitch became a handy place for jacking up the whole rear end of the car when necessary (Subaru was dead wrong about this part too, though in actuality I realize they have to assume that some people are like you, and can't figure out how to do such things properly so their official stance has to be "don't even try"). Related to that, near the end of the car's life it got rammed from behind hard enough to drive the drawbar of the hitch nearly through the bumper of the other car (the other car was mid-size, but large by today's standards, and bumpers were made of steel in those days too, not plastic). The drawbar of the hitch was slightly twisted by the impact, but there was no damage whatsoever to the body of the Subaru (I checked extremely closely - NOTHING on the car got even the slightest bit bent).
My "speculation" about the effect of a material's shape on its strength didn't just allow my car to tow light trailers and make it convenient to lift the back end of the car, it made the difference between the car being totaled in the wreck compared to having no damage at all, other than to the exposed portion of the hitch itself (that's a pretty good result simply from the "thoughtful" addition of about 25 pounds of scrap steel, huh?). But oh yeah, it's always better to put all your faith in the manufacturer and their official advice rather than rely on your own ability and common sense.