Good luck to both of you!
Feel free to ask/PM me with any questions. Snaps sound like a very good idea. I'd try to use plastic snaps though, so that the cold metal won't freeze you where it makes contact.
Another tip I can offer is to cut the center strip longer than you think you'll need, for fudge factor, and when combining with the side pieces, start from the top/face then work your way back to the neck/back area, then you can shift the down away from the excess, cut/tuck/topstitch it shut.
Also, cut the drawcord piece longer than you'll need (I use a 1.25" wide strip, but I'd suggest starting out with 1.5-2" wide until you're comfortable sewing tight seam allowances). I fold over and sew .25" of the ends to hide the raw edge then fold .25" of the fabric along the whole length and iron flat (inside/cired touching inside/cired). Then I sew the raw edge of the drawcord strip to the raw edge of the face opening, with the strip on the outside, with outsides (non-cired/calendared) touching. Then, I fold over the strip to the inside and sew over the .25" fold I made earlier (thus housing the raw edge).
It's a little hard to visualize, but you'll be able to figure it out.
***MOST HELPFUL/USEFUL TIP: when starting/stopping a stitch, put a small piece of newspaper under the fabric (so the teeth at the bottom of the machine that move the fabric grips the news paper) and sew through that as well as the fabric. Since UL fabric is so light and thin, it has a tendency to get pushed into the machine and you'll get bunched up seams where you start/stop (especially when close to the edge of the fabric with tight seam allowances and corners). The newspaper prevents that, and you can just tear it off. News paper has a "grain", it'll tear more easily down one way vs the other, so make that grain/tear direction parallel to your seam direction and it'll be much easier to tear it away.