My Anet A8s frame was skewed because the laser cut parts had non straight sides. I replaced most screw connections with these corner brackets and now the printer finally prints straight.In total i used 12 of these.
Print the corners laying flat with the flat, wide cross bracing on the bottom. It should print fine without raft or support. The resolution is more or less unimportant but finer layers usually give flatter surfaces.
Use these corners everywhere where the frame slots together at a 90 degree angle. Be aware of the stuff mounted so you do not end up blocking mounting holes.