Options for adding an additional valve to irrigation system
It looks like you have some &quot;wiggle room&quot; - the existing valves could be pushed closer together, though it might require rebuilding the manifold (branches off the supply line).
My 7-zone system is indeed in a single rectangular box. For ease of identification, I would recommend (at least) labeling each zone valve and if at all possible, re-number from 1 to 8 consecutively to avoid future confusion.
For your zone 8 to point the other direction, you should either route it around after it exits the box or dig deeper and route it underneath the valves. The valve should be in-line with the rest of them. Use care not to exceed the maximum bend radius for your tubing and do not kink it.