Has anyone every used kayak/rafting-style dry bags for sand bag training? Sierra Trading Post has them for $13-49, depending on the style and size. Anyone experiment with them? Worth trying? To me, they looked like they could be ideal for cardio-oriented workouts because of their size and shape. They're relatively inexpensive, pretty burly (19 oz vinyl), and they have a great roll-and-clip seal that would prevent leaks. On the downside, since they're vinyl they might works better as liners inside a canvas duffel. On another note, this is my first post...thanks for welcoming another nOOb and for all the helpful info I've been digging out of the forums.