buy 5yds of fabric (1.1oz rip stop is the cheapest and can be found about a million places. Don't pay over $5yd)
buy 2.5yds of insulation (climashiled XP 2.5 for 40-50* COMBAt 3.7 for 35*ish and XP 5oz for 20-30*)(thur-hiker.com is only place i know of right now)
Cut the fabric in half (2x2.5yds)lay the two pieces on the grond one on top of the other
lay the insulation on top.
The top layer of fabric will be the shell and the bottom layer will be the liner
Measure your dimensions and pin it all together then cut it out (way faster than measure cut then pin as everything is already together and you get a clean cut line)
Add draw cord channels and lines at the top and bottom. If you want under straps slip them between the layers of fabric and pin them in place. If you want snaps add the gross grain pull tabs now, or add a zipper at the bottom to create a foot box.
sew all but 8" closed, pull it inside out, if all is well top stitch the 8" opening closed and go camping
This takes me under 3hrs. I have done a lot of quilts so plan on a little longer, but it is a very easy and fast way to get a nice piece of gear.