How do I set a custom field to be optional on more than one campaign?

I'm trying to make a custom field optional on some campaigns, but required on others.

Because it's a dropdown to select campaign on a custom field, I can only pick one campaign or all campaigns for a field. Similarly, I can only make a given custom field required or optional. I'd like to not have to create a custom field that's essentially the same thing for every single campaign on our site. Is there a way to do it with just one set to required and another set to optional, or do I have to make six different identical fields - one for each campaign on our site?

Thank you, Dam. It would be a nice feature to have. I now have to have 5 identical fields because one one form I want it to be optional.