I plan on using reasonably cheap powerbanks (like the Aukey Mini 3600mAh) with my own circuits, and – ideally – I'd like to have an appropriate fire extinguisher on hand in case I mess up.
I know that there's protection circuitry inside these things, but if there is a compact, cheap fire extinguisher that I can also use on LiPo batteries if the need arises, I'd like to get one.
This document claims that I do not need a class D fire extinguisher for Li-ion batteries, and that water or ABC will do (but also that you will need one for Lithium batteries – the non-rechargeable kind), but I'm not sure if that applies to lithium polymer as well. This Product Safety Datasheet for Energizer lithium polymer batteries doesn't inspire a lot of confidence.
Again, ideally this thing would be very small, easily available, and not too expensive.
I think a better way would be to isolate the fire, but there isn't much lithium in batteries.
Lithium polymer batteries shouldn't be too different from regular lithium batteries.
Another good way is fire isolation get some bricks or cinder blocks and make a flame "container", if you mess up the heat will go into the brick slowly.