: . . . The most likely cause was reducing the size of some of the
: pieces. ... The next most likely cause was probably poor material . . .

There's another explanation. Which way up was your boat? Boats are designed to carry a load in the middle. If you mount it on a car rightside-up as so many folk do then and tie it down at the ends then all the loads on the boat are reversed. Even if the boat survives, this treatment can cause hogging.

I always mount my boats upside down; they don't need covering that way, a frame shaped to fit the top of the boat is simple enough to make and the straps go over the boat midships where it is strongest.

I do put a safety line down from each stem to the car but that's so I can see if the boat moves; it's an early warning system but it will also keep the boat with the car if it does come loose, preventing it from causing an accident to a following vehicle, and there is not much tension in those lines.