Cons have different ideas of what is "safe," so be sure to check the rule of your specific con.
In general, con-safe would mean something out of plastic, wood, foam, or cardboard/paper. Depending on your skill level, this could decide what material you use. However, the basic idea is the same:
1. Create claws
2. Attach them to wires that will fit between your fingers
3. Attach the other ends of the wires to a handle that is hidden in your closed fist
This tutorial on youtube uses plexiglass, aka plastic, but the same idea could be used for other materials:
Backyard FX Wolverine Claws