Step 1: Materials
-A DC-DC buck converter. (1$ on ebay)
-A LED panel voltmeter. (2$ on ebay)
-An audio spring terminal. (0.40$)
-A potentometer, the same value as the one of the converter, mine is 10K (0.60$)
Step 2: No project box needed
If you use a transformer with a rectifier circuit, you'll need a different box.
As you can see un the pics, there's enough space for all the components.
Step 3: Electrical connections
What you'll need to do first, is replace the small multi-turn potentiometer with the big one, it'll make the voltage controlling easier.
Then connect the voltmeter and the spring terminal in parallel at the converter output.
Step 4: Preparing the case
Then glue them all to the case and cover the connections with tape to avoid short circuit risk.
I didnt have any knob on hand, i recomend you to use one for a better appearance.
Step 5: Final steps
Then close the adapter with everything inside and glue the case with a strong glue as epoxi or cyanocrylate.