And a 50 lb suitcase, in a 60 MPH frontal car crash into a solid object where the vehicle stopped in 3 feet (my guess from the front end crumpling), would exert over an average of 2000 lbs of force on whatever it was pressing against during the crash.Cargo netting would probably be much, much more secure than a sheet of plywood. As long as the attachment points hold fast, netting like that typically has a break strength around 900 lbs. of force. With factors of safety (under-estimating the official rating), it probably wouldn't break at less than 1500 lbs. of force. That said, if you're looking at spending that kind of money, I'd probably look into a roof box, hitch box (and hitch, if you don't have one), or even a tow-behind trailer.
Obviously, this would be a very extreme example, but if you're talking about securing objects during a crash, it's going to be hard to next to impossible to do this in a way that covers all potential crashes.
That said, a lot of solutions are better than nothing at all. And crashes like this are rare. At a minimum you want something that can secure the cargo under, say, 1 g of force, which may happen during emergency breaking events.