Get a piece of pipe insulation from the hardware store. They come in a variety of sizes, and you just need to get the size that will "wrap" around the bar, probably 1.25 inch pipe diameter. Cut the insulation to length, to simulate the distance you want to have between your hands, put it onto the bar, and wrap it with tape to keep it from falling off.
This insulation spacer can slide along the bar.....to the center of the bar when you work your kip cast to keep your hands the right distance apart, and can slide to the end of the bar after your turn. It takes about 2 seconds to move it into place, and less to move it out of the way for the next gymnast.