Hi
The thread depth is within the bolts on each four corner of the unit, the bolts have a threaded head, so the depth there is not the important part, if making your own bars just make sure the M5 bolt you buy to go through the bar and into the lamps bolt threaded head is long enough.
I would strongly recommend getting at least one hanger set and then do what the guy in the pictures has done and make two brackets to hold them at your own personal gap apart.
I would not advise spreading them more than 18 inches as the max spread of one unit is 24"
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