TeeDee went through this when he designed and made our down under quilts.
We saw several photos of various under quilts. For some reason all of the under quilts made up to that time used side to side baffles - no longitudinal baffles anywhere in any photos. In some of the photos if "appeared" as if the down was tending to sink toward the bottom of the channel and was slightly compressed. I say "appeared" since we were not able to examine any of the setups in person.
TeeDee finally decided on longitudinal baffles for 2 reasons:
- gravity compression towards bottom - again we couldn't swear that it was happening, but with longitudinal baffles the whole issue was avoided.
- no differential cuts needed - if using side to side baffles for an under quilt the question of differential cutting of the baffle to avoid down compression raises it's ugly head. Many people had been developing complicated computations and methods for cutting and sewing the baffles with a curve to conform to the bottom of the hammock so that the 2 shell fabrics weren't pulled closer together by the baffles and compressing the down. With longitudinal baffles the whole issue of cutting the baffles in curves is simply avoided and a non-issue. You still have to make the outer shell fabric wider than the inner shell fabric, but that is trivially accomplished