I have just made a 350 g shell for a fully baffled winter sleeping bag out of Pertex microlight. I made the footbox inner layer about 270 mm diameter and sewed that to the inner bag. I made the footbox outer layer 320 mm diameter and sewed that to the outer bag. You can't sew it all from the 'inside', so there has to be a small section which gets sewn from the outside, but that is where the down will go into the footbox anyhow.
One Planet in Melbourne, Australia are filling it with 650 g of 800 loft down for me. I posted the shell off today, so I can't show pics yet.
Anyhow, adding a footbox is pretty easy compared to sewing all the baffles in place!