1. You can use one or two elastic straps (1/2") to hold the quilt under the pad. Velcro is a messy solution.
2. I prefer to put either velcro or zipper to fold the foot vertically and continue a couple of feet up the sides. No foot box; the vertical fold makes plenty of room for the feet. Velcro makes a decent closure, but it has a fiddle factor. A #3 zipper is easier to use, but you will need a draft flap at least for the vertical part.
3. Taper the pad so it fits into the foot section.
4. Take a triangle off each 'shoulder', leaving the middle section about 24 inches long and going down the sides 6-8". It saves weight and bulk and you won't miss it. Run a drawstring along the central 24" section. Put a snap or button on each end of the 24" section to make a collar that snugs around your neck.
5. Reconsider the dimensions. I am about 6'2" and a side sleeper. I make my quilts from 76" or 78" long and 56" wide. The foot sections are 36" wide, tapered from a point 24" from the bottom. I use a 20" pad for ground sleeping and a 30" pad for hammocking. The extra width of the quilt is tucked under the edges of the pad and the elastic holds it there. I used to put two straps on my quilts, but now use only one, and rarely bother to hook it up.