I'd not thought of using thinner yarn. As it was, I had to twist it to get a thickness that would fit.
The reason I'd originally asked the question is that the top and bottom channels _look_ like traps, and the leak was only on the end of the camera (where the take-up spool is). The only reason I noticed it was leaking was I shot a few rolls without putting the case back on after loading; the leather case covers the joints between the body and the back.
@David Lyga - My grandfather used to close the back they way you mention, even when the camera was new. He was very patient and methodical with any mechanical device. I've tried to adopt those habits, but it is difficult.