See hivemonkey.com for the full project writeup, including electronics, code, Stryfe modifications, etc.
This is a case specifically designed to fit on the left side of a Nerf Stryfe and mount a particular homebrew electronics package, consisting of a Raspberry Pi Zero fitted with a Pimoroni Rainbow Hat and an ultrasonic rangefinder. The LEDs, alphanumeric display, and capacitive buttons are all available from the back of the case while the rangefinder looks forward to, well, find range. A slot on the right side of the case admits wires, since power, magazine sensing switch, and trigger switch are all located inside the body of the Stryfe.
The Rainbow Hat displays shots remaining in the magazine using the colored LEDs across the top, and displays range to target (in cm) in on the alphanumeric display. It also buzzes to let you know when the mag is either empty, or removed. The buttons on the bottom of the Hat adjust possible magazine sizes.
The mounting bracket on the case itself fits into the side cavity using the Hulker side-rail mount - secure it there with glue or whatever. The ultrasonic sensor fits nicely into the ports on the front of the lid; the support studs within the case match the mounting holes on the Hat and the top two holes on the Pi. The little extension on the far left side of the case leaves room the the Pi's microSD card to slot in without damage.
The v1 model of the lid has mounting studs inside which aren't necessary for the ultrasonic sensor, but might be useful for something else. The v2 model of the lid loses the mounting studs, and adds the rest of the Hulker siderail for cosmetic reasons and to allow a screw connection between the siderails.
The lid mates nicely to the case, but doesn't really lock in place enough that I was comfortable swinging it around with electronics inside, so I welded the lid to the case with a 3D printing pen. You could use glue or whatever. There's no way to get to the Pi's USB port to get on the network, so either be comfortable with your code or use a Pi Zero W. :)
Several different Thingiverse projects gave small bits of themselves to this Thing. Also pictured, though not included since I didn't remix it, is this great foregrip with space for a mini flashlight.
Hope you enjoy. And if you make this, and can write better code than I can - which you almost certainly can, because I'm terrible at code - please let me know, I'm pretty sure I'm doing some things the Hardest Way Possible.