You're going to void the FCC certification which the original laptop had.

Yes, I thought so, but does it apply to the housing of the antennas too? So the 3D printed example for the Macbook Pro would be out-of-spec? What about figuring out what replacement external antennas to fit to the Wi-Fi card's Ipex connectors, while staying within the certification? The certification says that no testing needs to be done, if the new antenna is of the same type and at most the same gain. I wonder what this means though for an internal Mini PCIe Wi-Fi card.

I'd say that the laptop was certified as a system: its case, antenna, and all. If you replace the antenna with something that's not an official replacement part or an accessory released by the maker of the laptop, that would void the certification. It's possible to take FCC certified parts and some cables, and create a system which doesn't pass FCC certification. That's how regulating bodies tend to think.

Let's look at your situation more broadly. Are you making a one-off for your own home use? Or it this going to be a production run? Or something more exotic?

Slabtop modifications are usually one-off modifications. But the question is maybe about how serious such modification really is. It could be that the antennas would be so low-power that moving them around does not change anything substantially.