Hi,
I build an eGPU with a AKiTiO Thunder2 and a GTX970 which is the most powerful solution you can fit into the AKiTiO case. With my 15" MBPr I can use that power on the internal and external monitor for gaming/rendering/connecting multiple displays.
There are many DIY solutions and most are based on the AKiTiO.
Check out the forum with most of the eGPU magic: https://www.techinferno.com/index.php?/forums/forum/83-diy-e-gpu-projects/
And some of my eGPU videos: https://www.youtube.com/user/Dschijn/videos