That is a cool pouch.
Here it is from a slightly different perspective:

As with identifying any prop in this film, there is a question that we need to ask ourselves: was it (a) a dedicated production-made item, i.e., a prop; (b) a repurposed military surplus item that has been production modified; or (c) a repurposed military surplus item that has not been production modified?
Assuming that it was neither option (a) nor (b) and is an otherwise commercially available and easily identifiable item with probable military surplus origin (as is the case in the N2B jackets), I conducted a brief online search with various search terms. I scrutinized numerous photos in an attempt to locate the pouch in question.
I haven't been successful, but I have some general speculative remarks to maybe help narrow down the search, if in fact it is a military surplus item and not only a prop or production-modified item.
It appears to be rubberized, which strongly suggests that it is a post-WWII item, as rubberization was characteristic of Cold War era combat equipment. A limited number of items during WWII were indeed rubberized, e.g., the early variant of the Wehrmacht gas cape bag, but rubberization didn't characterize equipment before and during wartime to the extent that it did afterward. Since the film was released in 1992 and production started before that year, it is reasonable to assume that the item could have been produced between 1945 and 1990, capturing the entire Cold War period.
The materials, construction and dimensions of the pouch suggest to me that it may have been a surplus gas mask carrier. I cautiously make this assumption based on comparison with other gas mask carriers, e.g., that of the Swiss, which bears the strongest resemblence, in my opinion, and some others. Its original purpose may correspond to any number of things, however, such as pouch to contain medical supplies of some description, garments, personal provisions or possibly even ammunition. The guiding assumption being that the item it housed should remain as protected from moisture as possible.
Here's the Swiss gas mask bag for reference:

If we’re not successful in identifying the pouch as a military surplus item, if this is the case, or the pouch is only a prop or a production-modified military surplus item, it would be possible to fabricate it. It appears to consist of only rubberized canvas and a snap closure, with some means of affixing it to the body, which are obscured from view unfortunately. There may be a strap visible to the right side of the pouch, but in my opinion, this could be part of a garment or something else entirely and unrelated to the pouch.
In another photo from that scene (posted below), the prisoner in the far right frame is wearing two pouches, one of which (his right side) is at a glance almost certainly a black (or production blackened from olive drab) US Army canteen cover. I am not certain about the pouch worn on his left, but I could investigate. This lends some credibility to the assumption that surplus items were simply repurposed, possibly with some production modification.

Let’s keep looking.