Mike, Well I must have a really rare "edition" of parcel straps as the (believed to be stock) ones I worked on from my car had darn close to 1 1/2" straps and the slots in the mounting hardware were almost 1" wide. I believe the correct expression for fitting 1 1/2" of material into a 1" opening is called, to use a sewing term, "gathering". Just an addendum... the pack strap material I used is very thin - just like the stock parcel strap material. Not sure, but the automotive type store might be selling tie down, tow strap or even seat belt material. It's waaay too thick for this application. You might try a sewing, crafts or on-line store instead. Can't help with the Japanese cotton thread either.