Clear off the blown in insulation and cover the area you want to put the speaker with a nice long piece of 24" wide, thick pink insulation. The width works in 16" centered or 24" centered trusses. This will keep the loose insulation from falling through the hole you cut or covering the speakers and getting on to the cone. It will also allow the speaker to function properly. Most ceiling speakers will work infinite baffle without an enclosure.

Many manufacturers of mid to high end ceiling speakers offer some type of enclosure for their speakers. SpeakerCraft for example has enclosures designed for almost all of their models.