I am using the 8bit alexmos 2 axis controller.
The size of the imu measures 23x23 mm and the holes are 18x18mm, fitting 3 mm screws, so they must be around 3.5mm size.
I have mounted it on the backside with doublesided tape.
If you decide to alter the design of the camera mount, i would also be interested in another alteration to the backside, making it possible to use the backside connector for video i/o.
Find an example here