I picked up some 0.5mm as well when I was at the store...I don't know of an engine that Needs this small of a belt, (off top of my head) but I wanted to see if I could weld it. I did manage a weld but it was not pretty. I need to get my Good reading glasses from work and practice my weld method a bit more.
A bit off topic but it was Big Fun to belt up the QX6, it's seems Stirling engines are like Steam engines in that they make nicer "sounds" under load than No load conditions.