I used omni tape to form the footbox. It seems to be working well with no draft problems. The tape holds tight. I've used it with the 38" length at the foot end and that has finally worked out. The foot box can be put together in two different configurations; either as a tall/slender tapered shape that makes the most out of the total length of the quilt, or as a wider more flat shape that allows for more side to side room in the footbox.
You got it right with the hardware. I really like the d ring/mitten hook system. Once I find the best length the cord locks won't be necessary, and they get in the way on my back. The elastic cords with the mitten hooks are easily removable for use as an elastic belt for a poncho if needed.
The elastic cord makes it easy to pull the quilt tight along the back when side sleeping. No drafts, total warmth.
The head opening is 16". It's about 2" too much, but it's easy to cinch the sides to fit.
The last baffle at the foot end is tapered from 5" in the middle to 0" at the sides. This is to create the trapezoidal footbox shape when it's put together with the omni tape. I changed the amount of down in this last baffle to fit the actual amount of space (c/i). I'm not sure this is what you were referring to in the second to last question, please let me know.
The only thing I plan on changing on my next project is the footbox shape. I'm going to make all of the baffles 5" in width, then make a flat/wide foot box by keeping the main long seam on the bottom. This looks like the way the Zpacks quilts are made and will allow for more foot room from side to side. This seems like a better way to do it, but it will take an extra 5" baffle to keep the overall length.
Here are the two ways to put the footbox together with omni tape: