I have a (yacf) buff. And another buff (winter mode) and I've repurposed a couple of shirts (it's OK, they were already in the rag bag - does anyone else have a rag bag any more?).
I used the govt approved 15cm x 25 cm design instructions.
I find a cotton T-shirt sleeve is quick and easy - sew the cut end up and turn inside out for that professionally-tailored look, leave the other end open to add an insert if desired, knicker-elastic to the corners.
But I'm generally wearing a buff to go into supermarkets.