I'd like to add an inline battery backup to my computer desk and probably need to go with an ultra small setup or DIY something since I would like to build it into the 6x6 chamber on the back of the desk. You can see it here:
The fact you are asking were to start means you don't know what you are getting yourself into. To build an uninterruptible power supply requires in depth knowledge of AC and DC systems and battery charging systems. Separetly, these are easy to understand systems, but the integration together is not.