I made a quilt with lengthwise baffles and it worked out very well, except for a few problems I created.
The width shortened up a bit from the baffling, so you may want to start with a full roll width of 58" of whichever material you use. I had cut it for 50" plus seam allowances, but as it was I could just barely fit it around me without moving. It worked out, but I just couldn't move. I am 6' 180# for reference.
I did a differential cut for baffle height, with the baffles from side to side in inches being 1-2-3-4-4-4-3-2-1 with a 5" spacing between so that the majority of the down stayed on top. That worked out really well. Loft on top is around 5" and the part you stuff under you is much less. Very warm.
-Being able to shift down from top to bottom: it works out if you are wearing a down jacket to sleep. You can theoretically shift more down over your legs for the night. I never had to do this (partly because I overstuffed way too much), but it could be almost like a half bag.
I tried to taper the bottom, but cut it down from 50" to 30" instead of 40" due to a copying error. Way too much taper. It kind of looks like the bottom of a wafflecone when closed and is unusable. You may want to just avoid this step. Sewing the baffles into the angle at the bottom wasn't a problem, just stitch all the way to the end.
There are some good step by step processes on the forums I followed which helped out immensely. The lengthwise baffles are very long at ~80". I used some 1/4" masking/sewing tape to lay out where the baffles went when the top & bottom pieces were on the floor. Then I rolled it up lengthwise, sat it in my lap, and sewed the baffles (mesh) in bit by bit just pushing the completed part away from the machine. It's a long stitch, so you'll need a breather after each one. A beer made it go faster for me without quality control issues.
I have pictures of the all the sewing steps somewhere, but I can't find them after searching. They can explain this much better.