Step 4: Making the positive
The walls of the positive should be made from a nonporous material as to not stick to the silicone once it has dried. I recommend acrylic for this step, but really any nonporous material should work (glass, metal, etc.)
Once the positive housing is made, glue the tetris blocks to the bottom using a hot glue gun.
To prevent the silicone from sticking to the wooden blocks coat each of them in olive oil, or a non-sticking substance specifically designed for your exact type of silicone.