Guys,
I'm extremely tired, so I'm going to be a bit lazy here. I'm not watch an entire episode and/or pour over the relevant script, but ...
I am all but certain that the derelict Borg cube we see in "Unity" was completely powered down -- and yet, Starfleet personnel were able to beam inside the thing and still breathe comfortably.
One might argue that the cube we see in TNG and the FC/VGR models are different cube classes altogether. As such, that the latter
must have some kind of solid hull underneath all those bits and pieces is a foregone conclusion: no power, no forcefields. No forcefields, nothing to keep folks breathing inside except a tight seal around the whole ship.
As to the former, eh ... it depends on how you interpret the evidence: i.e., do we step outside complete suspension of disbelief and figure that the cube we see in TNG is, in fact, identical to the bigger-budget versions; or, since they look different and we treat visuals like hard empiricists, the lattice-work cube might have sections exposed to space where the other version doesn't?
IMO, the Borg would consider large areas directly exposed to vacuum -- and, as such, the need for energizing forcefields at all times -- an inefficient use of their resources. To that end, I respectfully point back to my earlier post

Underneath the tubes, pipes and whatever the fuck all that stuff's supposed to be, we DO see a solid hull in the process of filling back in. If you doubt me, watch the last 10 minutes or so of "Q Who?" to see the regeneration in action.
P.S. -- Wonky as it is, we see several Borg/former who were assimilated at Wolf 359 ("Unity," "Unimatrix Zero"). That lends some credence to the idea that Locutus' cube had a "holy shit, abandon ship!" escape vessel aboard, and probably a rather fast one at that since said survivors ventured deep into the Delta Quadrant in
considerably less than 4-5 years (? That's a rough guess. I can't remember exactly when VGR's third season was supposed to take place).