Lesson of today: Baking soda and white glue doesn't fit the bill when you need a decently thick filling material; I could only get it to work pretty much the same as the glue itself.
As a result, I'm trying spackling paste again on the dome, and coating the seams while I wait to get the paints, Mod-Podge, and visor material (once I pick it). Seems to be working so far, but it will require a lot of sanding to smooth it, obviously.