This model has many types of seams when sliced. These include external round/sharp, internal round/sharp, external overhang/underhang round/sharp.

It should be ideal for testing seam settings, such as scarf seams, because you will see your changes in multiple situations in the one print.

Source CAD

A STEP file is included, and the original Onshape document is here: https://cad.onshape.com/documents/ef247de872542826e004d2d2/w/5a9895e59f81530644aada48/e/373840bec59ccb89460f1a43?renderMode=0&uiState=65de732c4f3a8f62a2201997

Scarf seam testing

This model was used to try and tune scarf seams, an upcoming feature of OrcaSlicer. As you can see, visible seams can almost be eliminated.

For more details on what settings should be tweaked to get the best results, see this fantastic guide by Adam L: https://www.printables.com/model/783313-better-seams-an-orca-slicer-guide-to-using-scarf-s

Video: