I designed and printed a simple focusing device for my Full Spectrum MLE-40 4th Gen Hobby Laser Cutter. You can find it at FSL Focus Device on PrusaPrinters.org
The manual for the laser cutter shows a focus device cut from 3mm acrylic, which I originally used, but a thin piece of plastic isn’t really easy to stand up at 90 degrees properly (and won’t stand up on its own) so you couldn’t really tell if the focus was correct.
So instead I designed a triangular piece that could stand at 90 degrees much more easily. I chose a triangular shape rather than cylindrical or a rectangular cuboid so that I could easily slide one point of the triangular shape under the lip of the laser head.
Right now when it’s not in use it is stuck to the top of the laser cutter using some tape, but I’ve considered making a new version with embedded magnets so I can ditch the tape and stick it anywhere to the body of the machine.