The Problem: Your Raspberry Pi is just part of a project that incorporates other electronic components. You want to install everything into a single, larger project casing, but the Raspberry Pi has no secure fixing points.
The Answer: This plastic carrier frame. You can attach the frame to a prototyping matrix baseboard or your own custom PCB using four M3 countersunk bolts or double sided tape. The Raspberry Pi clips securely to the mounting frame, although it is easily removed.
This is also a great help if you're working on the bench with a breadboard job. You need the Raspberry Pi to stay securely in one place whilst you're delving in with your oscilloscope probes, or accessing the break out connectors.
The fixing points are positioned to coincide with 0.1 inch or 0.2 inch interval matrix board / strip board / veroboard. That's 2.54mm or 5.08mm. So, it's straightforward to drill out your matrix baseboard to 3mm at the fixing points.
This carrier frame provides minimum stand off of the Raspberry Pi from the baseboard. Alternatively, you can put nylon stand off pillars between the board & the carrier frame, and mount more components between them.