I have only worn a fly plaid once and then not for long. I have available a strong rare earth magnet that I salvaged out of a computer hard drive. I took the magnet and folded the fly plaid around the magnet. The magnet was position along the plaid so that is would sit on my shoulder and place the brooch where I wanted it. I then place a small strip of steel inside the jacket shoulder under the magnet. This held fine for the evening and did not put a hole in the PC jacket.

If I were to use this method long term I would sew a pocket inside the jacket shoulder for the metal strip and probably tack the magnet to the plaid.