Much like countless others on the boards, I tried doing Galvatron from War for Cybertron Megatron. The head is a custom cast sold on ebay from a guy named Arteloko Customs (great guy by-the-way, gave me some tips on how to connect the head to the Megatron figure).
It looks big compared to Megatron's, but that's because War for Cybertron Megatron had a disproportionately tiny head.