LiPo charger module for 5.2V/2-3A load
You may want to look into LiPo packs that are used in many RC applications, such as the ones available here. These packs consist of multiple cells wired in series, which you could then regulate down instead of boosting. These cells typically have a very high discharge rate, much higher than you'll ever need to power a Pi, and are available in some very high capacities. You can also purchase chargers for your cells here, and since these products are targeted at RC hobbyists, there are  many pre-built options for regulation, monitoring, distribution, etc.