John, I can't directly answer the question for sourcing the original product, but I could offer a suggestion for fabbing your own.
There is a construction product called Fome-Cor. I used to use it extensively years ago when I was doing reno siding on old homes. It is a quarter inch thick foam with perforated foil on one side, kraft paper on the other. 4 feet wide by 50 feet long, and it comes in a 2 foot increment fanfold.
If you have access to an aluminum brake, like the 12' 6" one I carry on my work truck, you could cut this product to size, run it through the brake and create your own duct. One side would have an overlapping flap that gets taped together with foil or duct tape.
It would be flexible enough to sideways flex it to a trapezoid shape so it could be flexed to insert into the narrow opening of the trusses, then returned to a rectangle shape. You could further insert rigid foam blocks on each side, at the trusses, to help it retain its shape.
I know I could do this as described, but you need a brake to do the folds. The brake has to have a gradual cam lock system to put gentle pressure on the foam board, not a snap lock system which would crush it.
Just thinking out loud with this idea, I have always been one to create my own things when none are available...
