I've been working on a Pi based IP KVM system project recently, and needed to make some 3D printed cases for the Pi's. (In a nutshell this project allows keyboard, video and mouse connectivity over an IP network. Details can be found here- https://pikvm.org/ ).
I am fairly new to 3D printing having only been at it for a few months. I was intrigued because the price point for an entry level printer is below $200 and I wanted one for prototyping and to make parts for other projects. I use Autodesk Fusion 360 for my 3D CAD software. You can get a hobbyist license for free but there is a learning curve to overcome especially if you do not have prior CAD experience.
I found a case design for a Pi 4 which I modified a bit and also added some threaded brass inserts to serve as a way to mount the Pi in the case. The inserts come in a variety of sizes and can be pressed in to the case using heat.
I also found some tips to heat the inserts that fit Weller soldering irons. They come in different sizes to accommodate various sized inserts. Up to now I've used the soldering iron tip to set the inserts but these tips do a better job.
I printed mounting standoffs in the cases and then used the soldering iron and special tip to press the inserts in to the standoffs. This worked quite well.
I then used 2 millimeter button head cap screws to secure the Pi to the inserts.
The top half of the case snaps on securely. It took several attempts to get the tolerance of the prints just right so the snap closure was secure.
The additional connector on the top is for an adapter to feed HDMI video to the Pi's built in camera connector.