not going silent on you, just busy summertime.
i LOVE your budget bag idea. it's great, and you're recycling, which is near & dear to my heart. my neighbors who recently moved tossed a whole down comforter into the dumpster; i was going to fish it out in the morning to salvage the down, but the trash truck beat me to it! (lesson: dumpster dive immediately when the opportunity is presented)
i have never used ripstop to make baffles, but i can't see why it wouldn't work decently well. if you taper, i'd suggest keeping the center of the baffles at full height for a greater portion of the length. for example, if you have a baffle that is 40" long, have at least 20" of that baffle at full height (although a 3" high baffle will result in a pretty warm quilt!).
sew-through is much easier than using baffles, but of course you know that will result in a much less warm quilt. even adding a 1" baffle will make the quilt much warmer. i sleep cold cold cold, so for me even a summer quilt still needs to be baffled about 2".