I found online a used thunderbolt to PCIe case from OWC for a pretty good price. The plan was to sell my dads old Mac Pro I was using and still be able to do some GPU intensive tasks. Also I wanted to keep my 4k monitor working at 60Hz where the integreated GPU in my mac mini could only deliver 4k@40Hz. I found a pretty cheap GTX 670 to go with it and it turned out to work great.
Now I had to figure out a good way to power this. The idea was to find a way to get rid of the MacPro as a big power supply and still be able to use an external GPU. So I dumpsterdived an old 200 Watt PSU and removed all the wires inside. The second step was to solder 2 6pin PCIe power cables to the 12V lane and shorted the wire that detects if the PSU should be turned on. And voila! No access cables!
Then I had to find out how to get the drivers to recognize this eGPU, and that turned out to be pretty simple. There are only a few .plist files that needs to be edited and that was fairly easy.