I was thinking something similar yesterday on the way home but I havent quite got it all figured out.
Squads could reserve time , have a set time say like for their squad night, like making reservations, after a set time like 15 minutes they lose the spot. May not be workable.
You could also use something like a system message: The arena is unbalanced, 5 knights waiting to join. then give some time for people to switch countries to allow those waiting to join in. Maybe hand out perks to those that switch? May not be workable either.
Let squads have assigned bases that they have to defend to keep (may not be enough bases to go around) or assign a base to a squad when they sign in. not sure how that helps, its just bouncing around in my head.