i made a defense with this system...but i never finished it
I can give you the Map...it's unprotected and you can watch the Triggers

It was somethink like this:
If a Player creates a Pylon, it turns into a Cannon!
A Lurker will be moved under every Cannon.
Then, a Location will be constantly moved o the Lurker..after this, i disabled/enables the doodad state for the cannon in this location and gave the Lurker to another Player (because the Location had to move to every Cannon...)
And after Lurkers were given to antoher player, the Lurker were given back to the Original Player and it started again ;D
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it's kinda pointless, that lurker.
since you preserve the trigger only in the end of it, and center the location on the pylon before it became a photon cannon, so the location will stay where you need it in the whole procces and you won't need any extra units.
besides, it's impossible having a building built on a lurker.