OK, I had this plug-in charger going spare. I'd bought it to replace a lost power-supply for a car DVD player, but it wasn't up to the job.
Time to give it a new lease of life.
Step 1: Oh, It's *so* Hard to Make...
I lopped off the useless jacks, split the wire, stripped the end and soldered on the crocodile clips.
It was nearly as quick as it took to type this step.
Step 2: The Interesting Bit.
OK, so I have a cheap power supply (the charger cost me Â£2.99), so what?
Ah, because I can choose the voltage! It's a bit hard to see in the photo, but there's a sliding selector for anything from 0V (why??) to 12V, and a little recessed switch to swap the polarity as well.
One "problem", the voltage on the scale is wrong - a voltmeter across the crocs reads about 50% higher than the number on the scale. The working voltage is actually 4.5V - 17V.
It's not just a toy, it's been used in anger, to test the bread-board circuit for the version of Minty Beating Valentines Heart I'm part way through (https://www.instructables.com/id/EG7BX7AVJ7EXCFLX7K/ - I'm just looking for a suitable box to land a dragonfly on).