When I wrote "shower cap thing" I was referring to the Pro Wire ISF-147 Insu-Flate Ceiling Speaker Protector, which is a product expressly designed to keep blown in insulation out of in-wall speakers.

Insulation improves heating and cooling efficiency while lowering energy costs. Insulating your ceilings will cost between $875 to $7,350 for a 1,200-square foot area, depending upon the blown-in ...