I once had a 6 gpu rig made with wood, and what I did was run 1.5sq mm wire with rig terminal to each of the gpu, and the PSU, I put the rig terminal just below the top connector and screwed the GPU in place and for the PSU I place one above the backplate connector and secured it with a screw. I had all the ground wires in turn connected to a 4sq mm wire that was connected to the ground wire.

To be honest that was my first rig and was scared so went with that crazy setup 

Regards,
BLife