I'll try again: Except for leaving the quilt unfilled to markedly reduce the duties the OOP would face by importing a finish product, there is nothing custom about what the several quilt makers need to do. Except quote a price for an unfilled quilt.
Hungarian down isn't improved by being shipped twice across the Atlantic.
On top quilt: Comfort matters. My experience is that 40F / 5C of covering, rather less than 50mm of loft, is far more than enough at 0C if the bottom of a Clark is well covered with an underquilt. That depends, of course on controlling drafts up top.
OP might also check with companies such as RAB, based in the UK. I believe the microfibers they use in their better bags and quilts are all polyamid = nylon. But, with hi zoot brand names, like Pertex.