We use the Nail Board seam on larger surfaces. Sometimes it is not practical to use a piece of aluminum.
Flatten one side of the ice you intend to fuse with the Nail board. A circular pattern works well
Next, flatten the other surface
Flip one surface onto the other.
Fill a large syringe with cold water
Separate the ice with a screwdriver
Squirt lots of water into the seam using the syringe.
If your ice is below 15 F you will need to modify this step (add water more slowly)
Quickly remove the screwdriver & watch as the water squishes through the seam.
We used the Nail board method to create most of the seams in this sculpture for Nike