Everything was available at my local Ace Hardware, total price was just over $6.00.
(1) 3/16" x 3/16" Compression Union - Link
(2) 5/16" Compression Insert - Link
Step 1: Optional - Tap / add threads to the holes in the pack shell.
The holes don't need to have threads, however, I chose to do this. I'm using a 3D printed pack shell and used the fitting to cut the threads directly into the PLA by screwing them into the holes (from the inside of the pack shell). I wanted to mention this first because we'll be cutting the fitting in half and doing this beforehand is easier than after.
Step 2: Cut the 3/16" x 3/16" compression union in half.
There are several ways to do this, please be safe and wear proper PPE while using tools. I used a bandsaw, but you could put the fitting in a vise and use a hacksaw. Be sure to leave yourself enough of the hex center section to get a wrench on afterwards.
Be sure to deburr the parts afterwards.
Step 3: Cut ~3/16" off of both 5/16" compression inserts (the end without the flange.
Again there are many ways to do this. Please be safe and deburr the fitting after. Here's a cut fitting vs one that isn't.
Step 4: Install
Unscrew the compression nuts from each half of the fitting you just cut in half. Be careful not to lose the compression sleeve that is inside. Here is a quick reference for how everything will go together
First, take the threaded portion of the compression fittings and thread them into the pack shell from the inside so that the threads are visible on the outside of the pack shell. The hex section of the fitting should prevent the fitting from falling out of the pack shell.
Next, slide the 5/16" compression inserts through the compression sleeves.
Place the flanged side of the compression insert/sleeve combo into the threaded fitting and then screw the compression nut onto the threads. You'll need to hold the threaded portion of the fitting from inside the pack with a wrench or a socket (I believe 3/8") while tightening the compression nut.
Here is what you should end up with (5/32" red tubing for reference).