Right, if you're an authenticity nut (like me!) don't use a pennanular brooch. They fell out of use in the early middle ages and are not "right" for the great kilt period....

A bodkin is just a big, heavy straight pin of any material: iron, brass, bone, a sliver of antler, a thorn. Raiding Granny's box of old hatpins will often yield something perfect. These make great bodkins, too:

http://www.craftycelts.com/hair/lightpins.html