Save custom projection?

Is there a way to save a custom projection so that I do not have to re-enter it every time I use it? I cannot seem to find a way.

To save only the projection settings is not possible, but you can save the parameters for the whole reprojection.
Make only your custom projection settings and then save it. Then you can load it and the other parameters are still the defaults.
You find the load and save actions in the file menu.