Credit to Gerhard van Niekerk in Kimberly, South Africa (at the time of publishing)
"This is by no means 100% accurate, but the way I make seat harnesses. I use bookbinding spine fabric, cut to size and threaded through and fixed to the harness hardware. I then soak them in a mix of craft glue and warm water with a tiny drop of dishwashing liquid added to break the water's surface tension. This makes the fabric soft and poseable with the added bonus of keeping everything in place once dry. I use a tweezer to maneuver everything in place. Once everything has dried, I'll route the ends of the belts out of sight and give the whole thing a coat of clear flat and apply the harness name stickers on the shoulder belts.