Reddit mentions of Ultra-Small LM2596 Power Supply Module DC/DC Buck 3A Adjustable Buck Module Regulator Ultra LM2596S Compatible with Arduino by Atomic Market

A buck converter will step down the voltage and be able to handle the current draw of the Arduino.

https://www.adafruit.com/product/1385

https://www.amazon.com/Ultra-small-adjustable-Compatible-Atomic-Market/dp/B00TMFKJ9Q/ref=asc_df_B00TMFKJ9Q/?tag=hyprod-20&linkCode=df0&hvadid=198062682203&hvpos=1o5&hvnetw=g&hvrand=18113234875797766115&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9026803&hvtargid=pla-319814629648&psc=1

Ultra-small LM2596 Power Supply Module DC / DC BUCK 3A adjustable buck Module Regulator Ultra LM2596S Compatible With Arduino by Atomic Market https://www.amazon.com/dp/B00TMFKJ9Q/ref=cm_sw_r_cp_apa_i_Md0rDbDPNY7QG

The switch, or more precisely the circuit board for the switch, came from here: http://www.mausberrycircuits.com/collections/frontpage
Pick the one that fits your needs. You'll also need an actual switch; I grabbed a momentary switch from Radio Shack. Again, pick one that fits your needs. You can use a momentary, slide, toggle, pushbutton... the Mausberry doesn't care. You'll also connect the Mausberry to 2 pins on the Pi and download s script from Mausberry that executes when the switch is flipped (thus the orderly shutdown). You know how the Pi's red light stays on when you shut it down via a command? The Mausberry completely shuts off the power to the Pi, so no more red light.
I've had the buck converter for a few years from another unfinished project. It accepts up to 30 volts input, and output is adjustable via a tiny screw. I have mine set to output 5.15v. It will output up to 3 amps. I remember it came from eBay via a slow boat from China, but I'm sure Amazon has the same thing. It doesn't have to be adjustable, as long as it outputs 5ish volts.
The relay I think came from Adafruit. It activates (closes the circuit) when one of the Pi's power pins (i think I used pin 1) is hot. So if the Pi is ON, the hub is ON and vice versa. When activated, it allows power to flow from the buck converter to the USB hub. The USB hub then powers the SDR's and the WiFi dongle.
The hub itself is a cheap unpowered hub from the radio shack clearance rack. I stripped it down to the bare board and soldered wires to the pos and neg to make it powered.
I wanted a hub so I could offload the peripheral's power requirements from the Pi to the buck converter. I just don't think the Pi really likes pushing that much power to its peripherals.
The GPS/AHRS gets power via the Pi's pins as per the normal connection, but I had considered powering it from the relay.
The reason I went to this length for the power is because in the beginning, it was hard for me to find a 5v power source that could provide the necessary power via a USB cable. Either the portable battery was lacking, or the cable was lacking, or the 12v USB adapter was lacking. So the obvious solution to me was to accept ship's power (12v-24v) and convert it to 5v myself. From there it was an easy decision to split out the power and ease the strain on the Pi. I have 12v power easily available in the Mooney and I have the portable 12v ammo box (in the pictures) when I fly in other planes, so in my situation a solid 12v source made better sense than an iffy 5v source.

Caveat emptor: With this setup, you are no longer building a "cheap" ADS-B box, but rather a darn good, reliable one at a very reasonable price. And should something inside ever fail, it's a cheap matter of replacing the bad component rather than the whole setup. WIN! I am ever grateful to the software developers.

EDIT: here's a link to the relay: http://www.amazon.com/gp/product/B009T2M012?ref_=cm_sw_r_awd_HYZBwb5RRB29

This looks like the Buck converter. Turn the tiny brass screw to adjust the output voltage: http://www.amazon.com/gp/product/B00TMFKJ9Q?ref_=cm_sw_r_awd_Y4ZBwbC5J8VK9

I’ve got no clue how that would handle drawing in addition to the usb load.

Again, (for our purposes) a sub-$4 component can achieve the same results as that $40 one, assuming 5 V is sufficient.

This $8 one is larger, but more flexible, so that is probably the one I’d test with first.