The medium-sized ammo can arrived today. I removed the lid, lined it with one-inch-thick foam (that came with some hard drives), and squeezed in 12 batteries, giving me a total of 30 amp-hours at 30 volts, 900 watt-hours. And then it fit nicely into the pannier. I reckon that's enough for about 60 kilometers. The whole thing, including ammo can and on-the-road tools, is 10 kilos, which isn't much when rolling along (I'm 100 kilos and the bike another 25, so it adds about 8%) and it's easy to detach from the bike when it needs to be lifted over an obstacle.
All I need now, is some decent weather.