Summary

This design, based on the one shared by Taylor Stein on Youtube (https://www.youtube.com/watch?v=E0bhdr84FNU), is meant for Arduino Uno Rev1 and Rev2. It is also suitable for Rev3 although not all pin connectors become accessible.
Using M3 screws (26.2mm including head)

Print Settings

Printer:

Geeetech i3 Pro B

                                            <p class="detail-setting rafts"><strong>Rafts: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>Yes</p></div></p>                                                                                <p class="detail-setting resolution"><strong>Resolution: </strong>                     <div><p>0.2</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>30</p></div></p>                                                        <br>                        <p class="detail-setting notes"><strong>Notes: </strong>                    </p><div><p>I printed the model using a 0.3 nozzle:<br>First Layer Height: 0.3<br>Layer Height: 0.2<br>Layer Width: 0.51<br>Perimeters: 4 (I guess 3 perimeters could give better results)<br>Slicer: Slic3r 1.2.9</p></div>

Arduino Uno case