On eBay there is a power supply PCB that includes a voltmeter and a ammeter. I connect a laptop power supply and a small LAB power supply is done! I desolder the trimmers of voltage and current limit and connect two same potensiometer. I use a 10 turn one for voltage. There are 2 PCB joined with screws. I replace the screws with cables to increase the distance between the PCBs.  Note that cables or screws is used for connecting electrically the 2 PCBs.  I desolder the two white connectors that exist in front. I use the holes that exist in the front PCB to attach it to the box. I desolder the current limit LED and use an external red one. I also add a green power led with a proper resistor in series. I add 470uF / 35V 105 C and a 1 K 1/2W to the output to reduce the produced noise. As I primarily use this to charge lead acid batteries and I add a TO220 double diode from an surplus PC power supply to protect the circuits from voltage that returns when mains power is disconnected. The common cathode is connected to the positive input of the PCB. The one anode is connected to the positive output of PCB and the other to the positive output of the laptop power supply. Take care to isolate properly the diodes from the chassis that you can use for cooling. Also take special care to isolate 230 mains cables !!!. With the current limit potensiometer you can charge a battery limiting the maximum current. Avoid drowning continuously more than 2 A. With a laptop power supply it can give about 18 V maximum. You can of course use any available power supply less than 30 V giving at least 2 A.