Apparently conventional soft box isn’t easy to carry. If you want to use it outdoor, the iPad app named SoftBox Pro should be able to help you.
SoftBox Pro is a professional soft box simulation app developed by Greg Wu, and designed for iPad. The photography app features 15 different soft box shapes, 15 different grids and patterns for the best detail in reflection, 8 levels of brightness adjustments, and simple user interface. All you need to do is to select the most suitable settings, and focus on your shooting.
SoftBox Pro with a size of 2.2 MB is compatible with iPad running iOS 4.2 or later. At present, the iPad app is available for free. If you’re interested, jump to App Store for more details.