I'm assuming the 48oz is about the same diameter as the 32oz, but taller.
LowePro makes a neat mesh one for their camera system. I agree on the Nalgene brand one for a sling type. It has a good sized pocket for your PSK stuff too.
Maxpedition makes several bags with Nalgene pockets. I picked up a small sling-style backpack that has a bungee cord arrangement on the outside that holds a Nalgene perfectly, leaving the innards for day hike and PSK gear. It would make a good mini-BOB. Add one of the metal pots sized for a Nalgene and you have a start on a nice kit.