Wow. So you are serious... If you haven't already figured this out, I shape and make surfboards. Making the whole board, the first time, you learn a LOT. But you learn a little bit every time you make one. Same for every thing else I make (I've completely redone our kitchen from the studs out). Get a process draft out of the first one, and continue to modify it as you make more. PROCESS is key to reproducibility; and if you are selling them that is what you are aiming for. Sorry, you probably already know this...Off soapbox.

Have you figured out how to calculate/estimate R-value yet? With the mesh tubes, insertion would be easy by pulling a line through first. But you would have cold spots between each "tube". Might be fine for a first pass though. The loose down sounds like a potential nightmare to me, given the nature of your baffle system. I suppose you could put mesh around your valve, but the down will still have a tendency to drift around. Probably already thought about this but might be interesting to get your hands on a "destroyed" down mat and cut it up. A little reverse engineering if you will.

I'm also thinking mesh baffles ...