Terrain joining

I have multiple terrains in my game and I would like to find an easy way to join them together because so there are no seams or variations in height between the terrains. I know about terrain.setneighbor but am unsure of how it works. Do I place a script on each terrain setting its neighbors. I've tried that but it doesn't do anything. I want something that joins the terrains together, averages the height diferences and removes the seams between them.

Eric5h5 has a terrain stitching utility on his website: Stitchscape.

I don't know of any free alternatives (unless you do it yourself).