nice job... I really like your introspection and analysis, and your model has given me (and others)thought for concern.... a comment for those that might live in areas with a little higher heat index...my 72hr bag is a Blackhawk with 100oz internal hydration bladder I picked up off the prize table at Ft Bragg. The drinking tube makes it a lot easier to keep hydrated if you don't want to stop to remove the pack... my external water bottles..SweetWater filter bottle with nested cup, and stainless 1/2 liter as you observed are also difficult to reach.. regards Les