Insulated Windshirt, ~10 oz ~$80

Doable, why not?

I wouldn't get the same same size, though. Make the inner layer "your" size and size up the outer to avoid compression.

I thought about that but was concerned about two things, one was the length differences (sleeve etc), and the other was the sizing. For me I think a Large would fit, and still have enough space to compensate for the additional insulation without compressing it.

If the Large is "a little too big" for you then it should work.

The avatars of the two participants in this thread are crazy similar.

+1 on if it is just a bit big. If you are really pushing out against the fabric of the large when you wear it normally, then you will compress the insulation. But if it is even just a little baggy on you, the insulation should be fine.

Also, do you have access to a sewing machine? Is that why you plan to hand sew it?

If you have access to a sewing machine, I don't see why you can't just sew the insulation to one of the jackets are the edges (so the neck and waist for the torso pieces and the cuffs for the shoulders) for most of the places. Then places where the insulation comes in contact, like on the sides of the torso and at the shoulder, you should be able to hand sew the insulation pieces together with light looping stitches, overlapping the insulation just a bit, and avoid any real cold spots.