I'm a bit late to the game, but had some thoughts I wanted to share.

First, I think a Goosefeet underquilt would be a great solution if you are willing to pay for it. I've heard he has good prices (and great quality) for custom down equipment, but it is still going to be a custom piece of equipment. It won't be the same price as a mass produced item from China.

You could try hanging an emergency Mylar blanket between the bottom of the cot and the ground. As long as you maintain a gap you will get good radiation insulation. You would still have quite a bit of area to move air, so you won't be able to cut out all convection.

Another option, that would be quite messy in a tent, but would work out better in a floorless shelter is to use some locally sourced leaves. You could pretty easily make a skirt to go around the perimeter of the cot and then stuff leaves underneath it. A moderator on this forum that I haven't seen in a while (Phat) I remember once posted how he had a piece of cloth he hung below his hammock and then stuffed it with leaves for a free insulation you don't have to carry in.