Is it hard to sew that canvas-like bag material? What about the nylon straps?

Not really... I've made back/butt-packs (for carrying reloading tubes with paintballs) in similar materials with a far more complex pattern (re-engineered). Only tricky part is the padding, but that shouldn't be all that hard either.

Best part is that one could add a little more padding than the original case...

/Michael - who uses a Pelican 1200 case