This is a filament test swatch I designed. It has a spot for the color of the filament and any special print settings. Multiple should be able to stack on top of each other and interlock. You can put a dowel through the holes to keep them in place. You can also use the hole to attach the swatch to your filament to mark/label it. There are stripes on the model to test how well each filament adheres to the build plate and sharp corners to test warping. The overhang of the hole provides an easy overhang to test and the writing tests fine detail.

Update: Now added one for ABS and PETG.