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I can think of a few options but all will require some kind of form and are increasingly difficult.

First you would need to make a wooden form with the dimensions exactly matching the opening connecting points on the plenum defroster outlets and the metal inlets on the windshield channel. You would need two forms since the ducts are differently shaped and I believe overall end opening sizes and connection dimensions are different between them. A few pieces of lumber nailed together would work for this. I stress the exactness of the dimensions because the materials used in the first two methods would not have a lot of accommodation for any significant errors.

You could try to use a thick cardboard such as the original is made of but I found it is hard to buy thick cardboard in small quantity and working with that thick of material is difficult and gave me less than satisfactory bends and overall look.

I think an easier first option would be to do sort of a paper mache thing only use thin chipboard or cardboard instead of thin sheet paper or newspaper. It would probably take a few layers of the cardboard soaked in a mixture of half Elmers white glue and water to build up a thickness close to the original. Make it in two pieces like the original either with overlapping halves stapled or pop riveted together such as the factory did or with halves meeting exactly in the middle. If you did overlapping you would need to watch the end dimensions while the middle meeting method could be fastened together with an overall wrapping of a few layers of a thinner paper mache to make a solid smooth duct when both sides are complete. With either method paint to a flat or semi gloss black color when dry.

Second option would be essentially the same only use thin ABS plastic sheet heated in an oven until it is very flexible and can be molded over the form. That is a bit harder to do as it is hard to handle when hot and it cools quickly but provides a thin hard and lightweight duct. It would still need to be done in halves and joined somehow. ABS is what I used when making the large single defroster outlet for my AC evaporator box.

Third option is much more difficult and would require the same inner form as well as a larger outer form. The outer form would need to be larger in all dimensions so as to form a hollow space to the inner mold of about 1/4 to 3/8 inches all around where you could pour in a closed cell foam to make the ducts. All forms would need to be smooth and carefully sealed with shellac or varnish and then copious amounts of wax or mold release used when the varnish is dry. I did this when making the air outlet ducts for my AC conversion. Smooth-On makes closed cell foam casting materials good to around 300 degrees.

Here is a photo of what I made. Originals are on the bottom and are foam rubber air ducts with the center defrost duct in cardboard. Top is the repros with Smooth On castable foam ducts with heated and molded 1/16" ABS sheet for the defroster.

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Fiberglass?

I'm not certain ABS would be resistant enough to heat. Might need to do something like PEEK. Or riveted steel.

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Fiberglass might be an option but it needs someone with more skills in moldmaking than I have to do it. I tried it on the defroster before I finally used the ABS. Found I did not have the proper equipment and the fiberglass sagged too much in the simple mold pieces I made of the original cardboard duct outside shapes. The defroster duct has several elevation changes and was a disaster trying to hold the final form and shapes long enough to dry. The ABS cooled to shape almost as soon as it was laid down.

If someone can carve an exact shape with inside dimensions that the fiberglass could be laid over it might work. Other downside is the smell. I had made other fiberglass pieces and the stink was still fairly high a year later. Heat might make that come into play.

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As I recall, the stuff I used seemed to deform and become soft at around 250. I honestly don't know if the heater/defroster could get that hot with airflow at that distance from the core but it is something to think about. All in all I think the paper mache approach would be the easiest to do on those two defroster ducts.

Do you guys remember that plenum that QuietRide mistakenly sent me a year ago for 51-54 models?
I am fairly sure that was made from ABS plastic.

I was planning to make a mold this week for shaping "something" around. I think I could form the 2 pieces of abs as straight sections and then make angle cuts where I want the curve to be. I could probably use rivets to put the two halves together. Of course, it may take a couple of practice tries to get it right. I can get five, 12x16 sheets for $23 on Amazon.

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The core temp if always going to be less than the engine temp. As such the temp downstream of the core is always going to be less than the core.

But a word of caution, they stopped using ABS and switched to PVC in exhaust venting for high efficiency home furnaces due to ABS off gassing when it got hot.

But I'm sure we are taking higher Temps there then what is sent in an automotive heater situation

Good job

For a 3d printer..

You'd have to use higher temp filament for it to hold up

I found this on Wikipedia for ABS plastic, "at higher temperatures at or above 400 °C (750 °F) ABS can decompose into its constituents". It also states
"Ultrafine particles (UFPs) may be produced at lower temperatures (such as in 3D Printing)"
That is still a higher temp than an automotive heater.