Adjustable power supply based on an LM2596 module. Not really a true hack but something dirt cheap and simple.
AC adaptor (30V @ 0.8A output)
cheapo LM2596 module (there are plenty of them on fleabay - they usually cost just 4$ including shipping)
some wires and male headers
I am well aware this is FAR from a real hack, but who said you have to build everything from scratch when the Chinese sell these for just 4$ ?
Just 4$ each
the higher the output voltage the better (best to limit it to 35V max)