Here is the form disks for bending the 1/8" plywood and 1" foam around to make the thrust duct. Its two 30" disk made from 1/2" plywood screwed together with 2x4 spacers between the disks. I cut the two disks with our router, this way I have two perfect circles that are the same dia.

Now I have the 1/8" plywood wraped around the disks.

I decided that I would make this duct different then I have in the past. Instead of cutting the foam out in an arc shape and then sanding the profile on the arc I just wraped 2 layers of 1" foam around the disk and the duct down lathe style. This is the first layer. I cut the slits about half way through the foam every inch so it would bend without breaking.

Here I am making sure the foam will bend around without any problems.

Next I glued on the 1st layer of foam. I used a wracheting strap to keep it bent around the disks while the epoxy was curing and I screwed on some pieces of wood where the foam didn't want to stay down.

Here I have the 2nd layer of foam glued on, I glued it on the same way as the first layer, its just smaller.

The next step was to sand the final shape on the duct. First I rough sanded it with a belt sander and a palm sander. Then I made a simple Treadle lathe to do the finish sanding, this way the duct would be pretty much the same all the way around. Here is a short video clip of me turning the lathe. I just spun the lathe up with my foot and then used some drywall sanding screen to sand the foam. The lathe worked really good.

Here is how I held the treadle thate to the work bench.

Here is the leading edge after I finished sanding it. The white stuff is light weight spackle that I used to blend the plywood into the foam and to fill the slits.

Next I had to fiberglass the duct. The duct tape is there to keep the fiberglass from sticking to the plywood forms, it would be kinda difficult to get the disks out latter if I fiberglassed everything together LOL.