Whatever suits your eye, holds the guitar properly and is comfortable.
I've used woven belts with the metal parts removed and leather tabs sewn on the ends, hand woven straps made locally, mandolin straps, cheapass guitar straps from children's guitars, braided hemp strings, etc.
One of the gits in my photos has a horrible purple velour strap that was on the guitar I scavenged parts from...guy that bought it loved the thing, go figure.
I use ordinary leather adjustable guitar straps prpoerly designed for the job, place the strap buttons on the neck and the end neck of the box, works for me (-:
I like using a telephone cable, which is just something I have laying around. I tie it onto the neck near the nut and to an eye screw at the tail end.
Not sure what the best one is, but I fully plan to steal my daughter's Rainbow Loom one night and make one using this.
LOL promises to be colorful if nothing else.
The lady I get woven straps from locally uses what's called an Inkle loom. Pretty neat to watch her work and the straps are strong.
My wife is a weaver (she teaches rigid heddle weaving, among other things, for a local yarn shop,) and is gearing up to make me a strap or two. She has a class to give next week, then she'll set up and get it done for me.
According to her, inkle weaving and card weaving are probably the two best methods for making a woven guitar strap, but the Rainbow loom should do it just fine too.
I liked Mike Orr's suggestion of using your old neckties. When you run out, get 'em from friends who don't wear ties any more.