foot box construction

So the down is ready, the material are bought, now for the design. The basic dimensions are going to be copied off an existing summer bag. The foot box is open for ideas.
My goals are a summer bag that can convert to a quilt. So I want a zipper but I want it to completely open up for real warm nights are lounging around the house or travel, which in my current state is what it will be used for the most.
I just sat in two sleeping bags. One the zipper ran down the side and down the bottom. No real foot box, but I realy liked the ability to completely zip it open to form a quilt. The other was a "normal" bag with a 3/4 zipper and foot box. I definately want a footbox. Much more comfortable.
My two options IMO are to make the zipper extend to the end of the side and use a drawcord. I've seen this done on this website. My other option is to make a foot box and extend the zipper though the middle of it or the bottom of it.
Any thoughts about my two options or another option?
Thanks,
scott

Have you considered using a cinch cord like the Exped Wallcreeper or the Feathered Friends Rock Wren? By using a separating zipper you can have a flat quilt, or quilt with a foot box (zipper open and separated with cinch cord forming foot box) or close both for a full sleeping bag when needed.

Thats what I was talking about with option one. Have you tried this? It seems that it would require the bag to be longer and I'm not sure how structured the foot box would be then. If I did this I would stick a "bag" inside the hole to give it shape.
TB

I own a down Exped Wallcreeper (caught an old model on clearance and it was nearly as cheap as making my own would have been). It works in all three configurations and also as long vest with a hood thanks to the zippered armholes. In order to use your creation as a flat quilt, you are going to have to have the full length of you body plus enough to keep your feet covered as the quilt goes over your body -- this is ample for the construction of a foot box when using this design.