Just thinkin', Anto...
If you could put up with the hassle, perhaps your niece could sew up a waterproof cover for a non-waterproof bag...a la Topeak. All it would take is some waterproof nylon, some cord and a cordlock.
Meanwhile, I'll keep watch on the local Craigslist. There was a Axiom Monsoon waterproof HB bag on there briefly just after Christmas for USD$10 -- looked new. If I'd only known then....
All the best,
Dan.