Nalgene makes a sturdy looking rectangular 64 oz bottle, and a Platypus-like 96 oz soft-sided cantene, FWIW. Both are available at rei.com.

I've been scared of these hydration systems with their complicated hose and valve assemblies that seem like bacteria magnets. This failure mode is another thing to worry about. I've never used a hydration pack so I'm still wondering what I'm missing. I've never had much desire to drink and walk at the same time. With a waterbottle, I stop, get the bottle out of the pack, drink, look at the scenery and generally rest up for moment, then get going again; no problem. Some people use belt or lumbar pack thingies that hold a small bottle for easy access in an outer mesh pocket (stop and refill from larger bottle when needed). That seems like a usable system too.