I'm big on authenticity, but I wear a penannular with my greatkilt, because it was the first thing I forged in my home smithy.

If you use a penannular, make sure the circle is big enough around to accommodate the fabric. Likewise, a bodkin must be long enough to keep the material from slipping. Penannulars are more secure, I think. They are the medieval version of the safety pin, just to early in period.