Ian's solution is very handy, because not only is it permanent; it also transfers itself to your other Apple devices, so once you have set up your keyboard shortcut on your Mac, it's also available on your iPhone.
I keep all sorts of junk in k/b shortcuts, such as sharp, flat and natural for music posts accessed by adding a forward slash to the word.
There is a further method for adding characters directly by using Unicode Hex Input as a keyboard entry method. Once added, you can type any character by selecting "Unicode Hex Input", holding down the "option" key and typing the 4 digit hex string for the required character. Example, to type 1/3 use $2153: ⅓