The deployment zones are created automatically, and as far as I can tell you cannot manipulate them.

I sometimes am unsure of where the deployment zones will be, especially if I start with a blank map, so I sometimes place a small marker like a grave or tree on the map, then play a custom 4v4 on the map, and note the location of the deployment zones in relation to the marker.

Maybe somebody with more knowledge can jump in, but I have not been able to customize deployment zones.

ichi