Basic buck converters found on Ebay are capable of 2A max W/O a heat sink 3A with a heatsink and this will require a piece of copper soldered or firmly clamped (with some heatsink goo too) to the board under the LM2596 and mounted to a 10 X 10 X 15cm or 4 X 4 inch piece of .060 thick piece of aluminum for aheatsink if you require 100% duty cycle... I'd recommend using an L shaped bracket fabbed from 2 mm aluminium and a 2 mm (I don't know which measurement units you prefer and the "Imperial" measurements are most familiar to me) so a .060 thick 1/2 by 1" bracket with a 6/32 screw mounted to a 4 X 4" .o60 or 1.5 mm is my thought.. You will also require a spacer under the device as the connecting wires are PTH type, this to prevent the other pads from accidental contact with the rest of the heatsink... and I use multiple buck converters... the trick is to divide the loads... Think of the primary power source as a high voltage, low current source and the wiring becomes simple. I typed ion the Google search box "Ebay buck converters" and the first or second choice was this.. which is identical to that I described in my last forum post on this subject...
http://www.ebay.com/itm/DC-Buck-Adapter-4-5-35V-3-33-5V-Buck-Voltage-Adjustable-Converter-/371039757167?pt=LH_DefaultDomain_0&hash=item5663b1636f
read the description at the bottom of the page.. 2A, needs a heat sink for 3A the price was $1.25 on a bid site,
(above courtesy of Docedison)