Actually you can do a donut bun with bra-strap hair. I have bra-strap length hair, and that's the only way I do my buns. What you do is, get a donut, then spread out your hair over the donut, then start from one particular place, and bring your hair around the donut and pin it in place. The thing is, I don't use scrunchies, to me, that makes the bun look fake. I use pins to secure the hair in place. I bring the hair around like how the donut is placed, so by the time I pin in the last bit of hair, every strand is in. The reason why I didn't like scrunchies was because after you put the scruchie on, all that hair was left shooting out arount the bun. When you swing your hair around the bun and pin it as you go, the result just looks better. And another thing, I don't buy the donut from the store! Actually I didn't even know then sold those. What I did, was got a clean pair of socks, cut the bottom of it, and rolled it into a donut. Worked like a charm.
hope this helps!