The greatest problem I see with this is not the construction but the fabric. Synthetic bags and down bags are made of different fabrics: down bags need to be made of down proof fabric to stop the tiny feathers escaping. They creep through that in the end too, but you cannot put down into a non-downproof outer. Not only will it escape into the outside world, but it will migrate through the lining and stab you, cover everything in sight in down, and get up your nose - literally!
Mad sewing witch!