This box:
Hammond 1591ESBK ABS Project Box Black
https://www.amazon.com/dp/B0002BSRIO/ref=cm_sw_r_cp_awd_yeFaxbRR3SAZY
The 7.5 X 4.3 X 2.2 one.
Couple of these:
StarTech.com 6-Inch USB 3.0 Extension Adapter USB 3.0 Port Saver Cable, Black (USB3EXT6INBK)
https://www.amazon.com/dp/B00S2N2Q4U/ref=cm_sw_r_cp_awd_zfFaxbB9HHEVK
They're actually too stiff. Get lighter easier ones to bend, but they'll cost a buck more each unless you wanna go super cheap.
One of these:
[26800mah Huge Capacity + Powerful 5.5A Output] RAVPower Portable Charger 3-Port External Battery Pack with iSmart Technology Power Bank for iPhone, iPad and Samsung Galaxy and More - Black
https://www.amazon.com/dp/B012NIQG5E/ref=cm_sw_r_cp_awd_LgFaxbXV141M5
Literally just dropped in the bottom of the box. Highly recommend getting a slightly longer box. As you can see I cut a slot to access the end of the battery and the USB ports. The USB connectors were a little too long to plug in inside the box and all the right angle connectors I could find were also too thick.
Any PI in a cheap case. Cheaper the better. You want it thin.
After the metal strip was screwed to the top, slightly offset so the screws wouldn't hit the PI case, stuck some sticky Velcro on that and on the Pi case. Pi and receivers are side by side and they just barely fit with the receivers turned on edge as you see from the one peeking out of the hole. Receivers aren't velcroed to anything they stay put fine sitting on top of the battery and squished against the Pi.
The power port on the Pi is toward the receivers and that limits them from coming any closer to "you" when looking at the end shot. The receivers bump into it. The flat ribbon cable you see is a USB-A to mini-USB that comes with that battery pack and it works good for powering the Pi.
I just stuffed the antenna cables inside for the shot but I'll probably find a way to wrap them up more after I see if I really want them sitting on the box in the back.
That's about all there is to it. It's kinda a "press fit" just whichever way around it would all fit. Another 1/2" in width and length would make it a little easier but it worked. A smaller or thinner battery would also help to leave some room to stuff cables, etc, but I really like that big battery (I use it for other stuff) a lot. I may get another one. It has three ports for output, a 1.0, 2.1, and 2.4A and a mini-USB for charging. It charges iDevices *really* fast on the 2.4A port. I have a really long USB to idevice cable that I can plug into this battery and run the iPad off of it, at the same time as the Pi. Only warning: The Pi and two receivers will constantly reboot on the 1.0A port. Use the 2.1A or the 2.4A for the Pi.
Whole thing could be prettier and if I could find screws that match the project box thread pitch in a thumb screw design (maybe at Fastenal) I'd replace those so I didn't have to dig out the Leatherman from the flight bag to remove the lid to pop a newer software version in the Pi. But for an experimental box, it'll work fine. (I'd also properly countersink those screws holding the metal plate if I cared to make it "pretty". I figure I'll only be using this thing for a year before something way better is out.)