If you're going to stack these boxes, you might consider anchoring them to the wall, for safety. A couple of simple anchoring bolt would do.
Actually, having Mini-me at the bottom would help, not the other way around. The added pressure at the pump inlet is favorable.
There's something in Airspirit's rig that you can adapt here: if you use copper, you can make vertical runs, in and out, and solder a few tees along the way, for however many PCs you want. Ideally you'd want a removable plug, so I'd go with a threaded adapter, like I was going to use on my HDD block:
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