I use this one for set and forget:
http://www.icstation.com/lm2596s-adjustable-buck-converter-mini-step-down-converter-power-supply-module-125v-p-1712.html
I use a couple of these for when I want to monitor output:
http://www.icstation.com/step-down-buck-converter-constant-current-constant-voltage-meter-power-supply-charger-module-p-2455.html
Remember that USB cheap USB cables can have a lot of voltage drop.