The easiest way to handle that relationship would be using Collections.
You can create a collection named "12345" and then put any related files in there.
If you plan to move those related files later, then I'd suggest using either Opus tags or Opus labels. You can use a specially configured Everything v1.5 along with Opus to later instantly find items with a specific label.
Here's a script that lets you find Opus labels with Everything. It also explains how you can configure Everything for instant results.
Let us know if you're interested in making it work with Opus tags, so I or someone else can give a more detailed explanation.