Print two of the size you need, using small 4mm or 5mm bolts with washers and nuts you fill the balls with dirt, good to mix a little bit of "Root Tone" or similar into the dirt, then clamp the ball around a branch of a plant you want to propagate.  

Wait at least two weeks before removing so that there is plenty of root material in the ball, then cut the branch off just below the ball to plant in the earth.  

This works for almost 100% of plants and trees.