Here's an article showing how one company makes its casks. I'm sure there are many other companies. It's much cheaper to make your cask as big and as thin as possible. After all, there's a free market here!
Considering the size of the thing, the walls seem to be paper thin, just enough to provide a radiation shield. The fuel goes in for 100 years, but it's not designed to come out. For that, they will have to split the cask, and put the fragile fuel bundles into something else.
So, in 20 years these Obama Barrels will lose their coat of paint, and the rusting carbon steel takes over. Not a pretty site. (Freudian slip!)