Not much of sea fog biomes over in Ukraine, so fog collectors won't really work (and wouldn't even fit the concealability requirements, unless doubled use with camo and drone netting). And that's basically the most advanced form of a passive humidity collector, while still requiring a lot of real estate space to be anywhere meaningful.
I guess the question is more for the trenches in Zaporizhzhia and SE Ukraine, which is more of a dry steppe ("Cold semi-arid") according to the Köppen? With mostly sand dunes on the spits as I remember from before the invasion?
Basically, you are left with what long‑distance hikers use – local sources with appropriate filtration, or hygienic wipes in the case of not enough local water sources at the position. And delivered barrels, but that's better for drinking use.
Wipes my be the easiest, as the volume and weight of them is the lowest, and nobody really cares about their CO2 and microplastic footprint when fighting to save your very country and peoples.
Pretty sure the frontline soldiers know the very best what works for them by now, as Ukrainians have so far shown so much ingenuity that it shames all the Western fighting forces.
So if you are trying to do this as a helpful project, I'd at first ask them what their needs are (I am assuming here you are not part of the fighting forces there, but of course I could be wrong, in which case have my apologies and Slava Ukraini!)
