Electrochemical manufacturing of small metal components
Nice idea, but difficult to do. 

The copper cations must continuously enter the "mold" electrode to grow there, but they must not be able to grow outside. 
If there are holes in the "mold" the crystals will grow towards them because there is where the cations will come from. 
A permeable "mold" would let cations in over all the surface, but nothing would stop them to continue growing outside the membrane. 

If the crystals tend to grow to the other electrode in the shortest possible path, you could place a very small electrode just above the center of the triangle of the other electrode. 
The shape will, however, not be very defined. 
Maybe in combination with a permeable "mold"...

Trapping of liquids inside the piece will be almost certainly a problem. 
You are not growing a single crystal. 
Many crystals will start growing all at the same time in different places of the electrode. 
When they join somewhere, they will not fit nicely together, so some liquid will be trapped. 
This occurs also when big crystals are made in a normal crystallization, especially when there is no stirring.