There is a question or two that may or may not be relevant. Will it hurt to loose the heat from your collector? IF your heat goes from the porch, through a wall into a place where the heat is needed, who cares if the duct is insulated? If it goes 5' in ambient in CHHHHHH...icago, you may not get any heat from your panel.
Birdhouse has a decent idea, although you will want as much insulation as practical. A 4" pipe surrounded by a 6" pipe gives 1" space to insulate. The BEST rigid insulation is ~R7 per inch.
At one time, there was available a fiberglass board that could be cut and folded. A few layers of that would work well. Or, one layer of foil/fiberglass and more layers of good rigid foam, overlapping. One 4' X 8' sheet of 3/4" rigid foam would make a lot of 4" duct cover.
Ron