On the 3rd loop, after she tells Downy and Sally to go to the Theater, she then goes and gets a shotgun (again!) and loads a bag with cartridges (again!). This bag ends up being the mask. We don't see her put eyeholes in the bag, or even remove the cartridges from it first. But we can assume she decides to do all this after looking in the cracked mirror, looking at rack of overalls, looking at the bag, and then back at herself,
Just watched this again. Immediately after the shot of Jess looking at herself in the mirror, we see her march out of the armoury dressed in the overalls. In this shot, she's carrying
two
bags: one containing the cartridges in her right hand with the shotgun, and an empty one in her left hand. As she climbs the stairs and then enters a corridor, she pulls this empty bag over her head and we see then see that it already has the eyeholes.
So: as you say, we can assume that she got the idea for the mask while looking in the mirror; and we can also assume that she made the mask from a second bag while dressing.
[The shot of the storage area in the armoury is very dark so it's difficult to see, but there's actually a
pile
of bags there: one is hanging invitingly over the top of the drawer (and this is the one Jess puts the cartridges in), but there are others folded neatly below it.]
"So I've got bullets, but no gun. That's quite Zen."

It's possible they're all the same bag, same shotgun, same overalls, etc. The ship resets at some point during each loop, so each Jess could conceivably be picking up the same objects.

But when Masked Jess is walking down the corridor, she has a bag with shotgun shells. Maybe there were more than 1 bag in the supply closet, since there were multiples of overalls and guns?

On the 3rd loop, after she tells Downy and Sally to go to the Theater, she then goes and gets a shotgun (again!) and loads a bag with cartridges (again!). This bag ends up being the mask. We don't see her put eyeholes in the bag, or even remove the cartridges from it first. But we can assume she decides to do all this after looking in the cracked mirror, looking at rack of overalls, looking at the bag, and then back at herself,