Thanks to Trudi for this Frugal Baby Tip!
I actually tried to make one of these diapers tonight, but ended up ripping the gusset part out as my machine won't handle the number of layers (it's a very old machine). I decided that since I'm partly ECing my son and he doesn't poop in his diaper, I really don't need the fancy gusset (even tho it looks great!), so went with a modified version allowing for fewer layers.
I basically did the back just like you described, made the little envelope fold-over part in the front, but I only sewed up the sides of the diagonal part and left the middle part open so I could stuff a doubler in there, kind of like it's a diaper wrap. The side flaps go over the doubler and provide extra layers where I need it most. I tacked a 4-5" length of elastic on each end, about a couple of inches from the front and the back of the diaper, lining it right up with the sewing
line between the thicker middle section and the lighter outer strips.
Then I folded the prefold sides in (like I was going to Angel Fold it) and sewed just outside the elastic to encase it. I did get some snap tape today so I'll have to see what that does. Next one (or maybe this one if I feel like ripping out), I'll make the folded down front part a little shorter as this dipe was on the small side for my baby. Also, I think I'll tack the elastic a little closer to the outside of the diaper to make enough room for a doubler (don't want it to be too narrow).