That's why I was partially concerned about that (19") air gap that you've made into a 4 leaf system.
It's a bit too late now to go back and remove the drywall and frame a system together that you could actually seal separately to keep moisture out of that air gap.
I can't imagine how much silica gel you'd have to use... maybe 300-400 lbs to last the next 10 years? And even then, I don't know how your construction is on that inside gap... e.g. the ceiling, walls, etc... Dumping a bunch of silica in there could actually cause mold/rot to occur if that gel were to draw moisture from outside.
Lining the whole air gap cavity with plastic would not be smart either... nor would say lining the lower area. You could again, cause mold/rot under the plastic.
I think you're pretty well done with anything you can do with that gap... including put anything like a stretched fabric across the gap... unless you can actually get in there with a nail gun to attach some sort of support framing.
Good shit ain't cheap, and cheap shit ain't always good.
The finished studio: www.darkpinestudios.com
Studio build blog; dm mobile.com A Rod Gervais designed studio