I guess I've heard of the egg shell method for many years, but never tried it. However, I've certainly boiled coffee in a can on an open fire, then dropped a spoonful of cold water in it to settle the grounds; works quite well.
And the instant coffee ain't bad, if you use the right amount or ratio of coffee to water, and the right amount is usually quite different from the instructions on the container.
And Egon, if I were you, I'd just have to find a different doctor.
This reminds me of an uncle I had who played golf almost every day for many, many years (a major part of his job was entertaining customers). So when he had a heart attack, his doctor told him to quit playing golf. So he changed doctors; went to one he frequently played golf with.