This thread is really good, kind of addicting to scroll all time.
Only problem though is the way it is structured. Its really hard to find the same post twice.

Maybe the admins should consider having a separate section for this !!>?

This thread is old, but still consists of the best suggestions for a Raspberry Pi UPS so adding my findings here.
Despite Pi being used for a lot of battery-powered projects, it's surprising to see that a well-defined UPS is not available.
While most of the above-suggested power boards will work fine, they lack form factor and customizability. I would rather take an open-source design and tweak it to my needs than just using a off-shelf black box.

So I am planning to improvise on this design called ["DIY Raspberry Pi UPS – An Uninterrupted Power Supply to Keep your Pi Safe during Power Failure"](DIY Raspberry Pi UPS – An Uninterrupted Power Supply to Keep your Pi Safe during Power Failure)

The author has provided both schematics and PCB layout so it should be easy to tweak the desing. The board is already designed to handle 1.5A (continous) and 3A (peak current) which would suffice well for my project. Though I could really use a software on/off feature like i should be able to shift between battery power and main power through a GPIO pin. Will share the design when I am until.

Until then if anyone has found better designs or features, please update me.
Power to you guys, thanks