How to prepare smaller amount of coffee than nominal in Moka pot

The Moka pots like Bialetti are very similar to the esspresso process except that the water goes through the coffee from the bottom in Moka's case.

They are designed to match the pressure of the water with the resistance from the coffee grind/puck (IIRC 1-3 Bar of back pressure). If you use significantly less coffee, the water can just run through the coffee compartment without extracting much flavor or worse, it can fill the compartment (replacing the missing coffee) and soak for too long.

My guess is that you'd need to reduce the volume of the compartment to allow proper pressure build up on the coffee. Perhaps roll a coffee filter into a ring shape and place inside the coffee compartment to take up the missing 5 cups worth of coffee grind and fill the inside of the ring with the now less coffee grind.

Again you may not be able to reduce the water all the way since it needs to be above the opening of the spout/pipe that descends into water reservoir. The solution here is to take the Moka off the heat when you're near your one cup so you don't dilute your Moka Espresso too much.

I use to use a 6 cups' Moka to make my morning coffee. I only load around 10 grams of coffee into the coffee compartment; fill the regular water quantity until the safety valve but when the coffee starts to come out I stop the fire once the outcoming coffee starts to become more lightcolored.

I serve and drink the result in a mug.

The result is not as good as with the regular way, but is quite ok. you can regulate the concentration by turning of the fire earlier or later.

I didn't try filling less water in the compartment, since I believe that it will cause the temperature of the brewing water to be bigger and the pressure to be smaller at extraction point - after all, the non-boiling water is used to be pressed by the water vapor that expands on the top layer of the water container... Writting this here makes me wonder if it would be a good idea to even try with a bigger amount of water than prescribed to increase pressure and decrease temperature, though i fear that that might be unsafe.

Edit:

After I wrote this answer I decided to further experiment: I concluded that using one or two sheets of paper coffee filter in-between the rubber rim and the top filter allows a smaller portion to be extracted at an higher pressure, thus getting a result that is more similar to brewing with a full coffee portion. Still, I stopped the fire once the coffee started to come out more light colored.

You should reduce both the amount of water and coffee beans. The exact amounts depend on your preference, but if you know how much you use for a full pot, divide both the water and the coffee beans by the same number. The ratio should stay the same.

Maybe filling up the filter cup with steel balls will help.