The question is why? Jay (Jayz2cents) has tested external GPU's and concluded that they are absolute, pointless, rubbish. If you need hard core laptop, you will buy hardcore laptop. Upgrading whatever you have doesn't have sense. I understand that you need compute power when you are not on your main station. All youtubers when they go on events do their rendering on laptops. But they get insane laptops. And you can survive with few hours more rendering for a short time.
I just don't see the point. I don't see anyone for any reason, buy existing external GPUs, let alone workstation overpriced GPUs. Even for Rampage V Ed10, 6950X, 128GB RAM, Titan Xp SLI, 5x 480mm radiator water cooled build, in biggest CaseLabs case i see the point. It is enthusiast build, and i want one. But why would anyone need external Titan Xp?