> Could I use some cheap synthetic insulation to make a flap?
The thought police will get you!
Why not: it's your quilt.
Hum, well, whatever works. I made my quilts wide enough that I don't need any straps for the top half, but I guess it's a bit late to say that :-)
What you might find is that you will soon get used to using the quilt and learn how to turn over *under* it, without disturbing it. We can do this now with two quilts over us - and they are slippery!
As for the snaps unsnapping - maybe it is better that the snap undoes, rather than having a strap rip off the side of the quilt in the field?