What I used:
- the Hellhammer cannon from the Baneblade kit
- a couple of plasma cannons from the space marine heavy weapon sprue
- the standard Leman Russ plastic kit
- Trim the battle cannon barrel and glue the Hellhammer cannon in his place, you may need to scratch a little bit the weapon shield if the barrel doesn't fit completely
- Cut away the shoulder and the hand on the plasma cannon, you can glue a piece of plastic where there was the hand, so nobody will notice if you don't cut it away completely
- trim the heavy bolters from the weapon sponsors and glue the plasma cannon instead, you can leave the plasma cannon cable attached to the weapon so it will look even cooler!