I am installing vapor barrier on a ceiling using poly, acoustic sealant, and a vapor-tight ceiling fixture (FN-426-C-V - pictured below). Note the ring of soft foam around the circumference of the fixture. Everything seems fairly straightforward about the process, but I'm not sure how to marry the poly to the fixture (or whether I even need to). I'm assuming I need to stay outside of the box, and imagine that I must need to fasten the poly to the outside for the box using tuck tape or acoustical sealant, but I can't find any information pertaining to the installation of this product. What is the best code-compliant way to fasten poly to this fixture?