Summary

Searched high and low for the correct size.

24.5mm wins the prize

Fits plastic marble runs and pumps

Print Settings

Printer Brand:

Creality

                                            <p class="detail-setting printer"><strong>Printer: </strong>                     <div><p>CR-10S</p></div></p>                                                                                <p class="detail-setting rafts"><strong>Rafts: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting resolution"><strong>Resolution: </strong>                     <div><p>0.15</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>30</p></div></p>                                                    <strong>Filament:</strong> AMZ3D PLA                Red

Post-Printing

Glue together two of these...

Plastic marble 24.5mm