Mind Device(?)
The device would send out a pulse which affects the minds of humans, making the device seem like it's the Overmind.
Perhaps also, humans that get near it would see the fake Overmind attacking them and their HP goes down, but this is all a fake: their HP isn't really going down, and if it goes below zero it will restore and they'll know for sure it's a fake.
Requires stage 2 (or 3) to build. Uses half of the points the Overmind needs.
Fake Reactor
It's actually a repeater built inside a fake reactor shell. It is unable to attack, and for the rest acts like a repeater. It can be used to distract enemies effectively.
Requires stage 2 to build. Uses the same points needed for a Repeater, and a bit more for the fake shell.
Either this, or you could go with the original idea and make it a holographic reactor. In this case, the bounding box would have to change. But at least then you could make it appear to attack aliens (although they won't lose HP, nor will they see them losing it like humans do).
-- Data