Hey!
I actually had to build mine for the same reason. I made mine from leather, but it would work with nylon too. It took me awhile, and the help of another set of hands, but I eventually built one that works amazingly.
Let me take some photos of it on, with and without my jetpack on to give you an idea of how to make one.
Basically you just cut straps and stick them together with a few buckles here and there.
So you essentially just make backpack straps, (with the center buckle across your chest) then another horizontal strap over your mid section, then have some buckles on your back that can clip into your pack.
You certainly have to play mental gymnastics as you make it, and try it on as you go, but you can get it if I could!