Try asking the sca_jml yahoo group for advice. I think they have patterns in their files section, but you have to join first to gain access. They are usually reenacting the Heian era and I'm sure many have made nagabakama before.
Also check out www.wodefordhall.com
. It's a friend's website, she does Heian Era SCA.
I imagine that to avoid having a seam horizontally, you'll have to cut your pieces very long and sew vertically (Japanese fabric is only about 35cm wide as it is, so that's how they did it), sewing the long strips together, then pleating those as necessary.