If your laptop has an actual PCI-Express slot on the chassis it's very doable. I built one for one of our 3D designers who wanted to use the Oculus Rift & Unreal Game engine for some design work on his laptop. Our Dell laptop has the slot on the chassis, so I bought an enclosure, an EXP GDC Laptop External PCI-E Graphics Card, an nVidia 970 and a power supply. Assembled it's not too big. It works quite well; some minor flakiness requiring unplugging/plugging the card but overall it was a success.