Many challenges use “Custom Map” as their setting. Any design saved on a given map name will be available on a Map with the same name, regardless of scenario changes. If a map is substantially smaller, you can place your ships outside your deployment zone.
Now one may think at this point "But if you move a ship out of the deployment zone, it gets removed entirely! Yes, it does.
But not if the ship is already deployed outside. Challenge 4587729 (my challenge, created as a sort of proof of concept) utilizes this, with a deployment saved from the largest possible zone size used on a much smaller deployment zone to place all of my ships outside the entire combat zone, causing enemy deployments to quite suddenly veer downward as the match begins.