Hey Joe:
Have you considered just giving the mag pieces back to the machinist and have him poke a series of lightening holes in the solid parts?
(Holes run through the inside to hollow material out that you couldn't see the entry points.)
A LOT less work and still sturdy compared to plastic sheet.
It looks like a good bit could come out of the "magazine well" and if the strip that runs up beside the "magazine-ammo-checking-viewing-window" is actually solid, a drill could be run through there, as well.
But this would be shop work cuz I don't mean a 5/16th (8mm) drill; I'm thinking like a 3/4 to 1 inch drill (19mm to 25mm).
"Alice" is just such an ultimate project, it seems a shame to "cheapen" her.
If you were planning to costume as Jayne and carry her a lot, I'd understand. But after you finish her, and the buzz wears off, you'll probably only handle her one or two dozen times a year.
Alice will sit or hang there, making you grin every time you look at her, but you probably won't play with her a lot.
Mike