In trying to find a dependable method to mount my small adding machine to my bike, I've came up with a method that can be used for multiple purposes. My suggestion would be to mount a metal plate covered with velcro (front and back) to your handlebars. Then, you could place velcro on bottom of various objects and be able to take them on/off your bike as needed.
For instance, small clipboards, hand-held GPS unit, clear plastic for maps/target counts, pens/pencils, etc.
I use a "handlebar" mounted counter which I now have mounted to this metal plate. Doing this, gives me the ability to support the cable better.
Below is image of method I used to mount the metal plate and my resulting setup.
