I bought a cheap pair of blackwatch flashes off eBay that had interfacing to provide stiffness. The only problem was that the things always seemed to wind up bending away from my leg and while they had plaid on the front, they were white on the back. I wound up finally just throwing them away because it looked silly.

If you are going to use interfacing, I think you should sew another piece of tartan on the back to completely hide it.