Charles
Beyond ensuring the top vent is not blocked, I see no reason not to insulate the lid to some extent.
The cover I made for my lid is pretty low-tech. I didn't know how well it would work so I didn't waste any time making pie-shaped pieces to form a compound curved 'hat' for the lid.
A band of insulation about 7" tall was wrapped around the base of the lid and the ends were taped together to make a snug fit. I have a thermometer mounted in the lid so I slit the insulation to fit behind its face.
Another band about 6" wide was then slit to fit over the handle and its ends were attached to the lower band with the foil tape. I wasn't interested in covering the lid completely and this arrangement doesn't block the top vent at all.
All the edges of the blanket are covered with the foil tape, so there's no exposed fiberglass. Since the jacket is attached to the center section by its machine screws and nuts, I may just paint it black to keep from going blind when the sun hits it and leave it on year-round.
I've used mine twice with the jacket on. Once for a long overnight butt cook and again during a four-hour bacon bash.
Hope I've explained this satisfactorily.
Ken