11" Macbook Air = 1.08kg HyperJuice 60Wh external Battery = 0.36kg

Comes in under your weight limit, will give you 10+ hours. Only disadvantage is it's a cumbersome / weird form factor, macbook plus external batt.

However, if you find a decent small bag, it won't really be that much of an issue.

http://www.hypershop.com/HyperJuice-External-Battery-for-Mac...