When I bought my first straight it was a Whipped dog sight unseen. with it I got two poorman strops. the one I've been using has served me well but I've always thought it needed a handle so I made one.
here's how I was holding it
just two small machine screws and some 550 cord. quick and dirty
Take a bit of tubing (like a cut piece of ballpoint pen) and put that inside the leather, and the 550 cord though that. That will help keep the strop flat and prevent cupping once the leather softens with the cord pulling on the sides.