You should get a Nook.
There are several variances to the Nook e-reader - mainly it's between the Simple Touch, Simple Touch with GlowLight, and now GlowLight Plus. Basically it whether you want a backlight or not or you want refurbed or new.
Let's see how it stacks up with your requirements:
Everything FOSS. Moot point. Every ereader comes with built in firmware and regardless if it's based on open source or not, it's going to have proprietary software to work with their hardware. Not only that, for DRM protected content, they are going to have to license the code that allows them to use that protected content. There's no way around this, but it really doesn't matter; where are you going to open source download ereader firmware that's actively developed
Must support EPUB and PDF. Nearly every E-Reader on the market, with the exception of the Amazon Kindle, supports the EPUB format and virtually all except for an early Amazon Kindle and a discontinued Sony Libre support PDF.1
Lightweight. This is purely subjective; what you can carry for 2 hours per day is totally different from what anyone else can carry for 2 hours per day. However, the Nook e-readers have consistently weighed less than 8oz (half pound) or less than 200g.
Long battery life. Again, very subjective as your use will vary in relation to everyone else's. However, based on the B&N spec page, it will last up to 6 weeks on a single charge 2.
USB connectivity. The Nook shows up as external storage whether on a PC or a Mac.
1 Wikipedia: Comparison of eBook Readers
2 Per B&N: Based on 30 minutes of reading per day and 1 page refresh per minute with GlowLight at 30% brightness and wireless off. Battery life tests conducted using specific units; actual results may vary based on device settings, usage, and many other factors.