I went through a few different bases before I found what I will be using.
Put a 25mm base on top of a 40mm base and trace it.
The on the inside corners add pins so you can put the 25mm base on top and have it not slide.
The Terminator Himself will goes on the small base so he will fit on a space hulk board.
But that small base stacks onto the proper sized base to use him in games of 40k.