It has been a lot of years now but my '68 originally had a thick insulation top well liner in addition to the canvas well. The second thick liner was on the trunk side and probably provided both temperature and sound deadening. That insulation was fragile and became a ripped mess over the daily driver years of camping gear and other stuff packed into the trunk. I got so tired of chunks of it falling off or hanging in the trunk that I finally probably removed all of it forty or so years ago.

The subject came up six years ago https://firstgenfirebird.org/forums/ubbthreads.php?ubb=showflat&Number=218628&page=2 but don't recall any more discussion or source for the stuff.

Anyone have any recall, source, or information about this liner?

I'll update my initial post. Here's my recent research to date:
1. I checked the Team Camaro Forum and it appears that the well insulation was probably Firebird only. Nobody responded that they have seen that part. One owner of a '68 all original with an original top has no well insulation on his car.
2. I cannot find the well insulation at any of the Pontiac repro parts vendors.
3. I have checked with two convert parts specialty companies. One replied that they don't have the insulation. The second said that their F-body sources acknowledge the insulation but that we would have to improvise if we want to replace it.
4. At least three of our forum's '67-'68 owners have or had the extra insulation (Bob S, hokie1984, and myself). So far no '69 owner has responded.
5. I have not heard of anyone with a restored 1st Gen who has replaced the part.

Another update:
The 1968 Pontiac Sales Manual has this mention of the insulation on page Q-2.
"Special Acoustical Features. Certain 1968 models have added acoustical features to enhance ride, comfort and quietness. For example, the upper-level ventilation system in Firebirds and Grand Prix permits the elimination of door control vent windows, for reduced wind noise. And, with the exception of convertibles, the Le Mans, GTO, Catalina Hardtop Coupe, Executive, Bonneville and Grand Prix models feature sound-absorbing, perforated vinyl-coated fabric headlining to effectively dampen noise.

In Convertible models, special cemented fastening of the top fabric at the center roof bow, plus a tighter seat at the windshield header and a taut cable in the side rail area, help minimize high-speed flap and wind noise in the top. Convertible also have a heavy cotton-fiber lining in the top storage well to muffle out noise in that area."

See: http://firstgenfirebird.org/1968-info/1968-technical-information/sales_manual/selling/q-2/