How to restrict project access to specific users

Summary: Is it possible to restrict certain projects to users beyond the direct assignment to the project? We are currently utilizing project security based on Business Unit assignment and data access.

Content (please ensure you mask any confidential information):

Specific projects are deemed confidential and need to be restricted to a subset of users. While you can restrict access to projects based on being assigned as a member to the project, our current setup is projects are viewable to the users BU they have assigned to their application security role. We are looking to restrict a subset of projects to only specific users, by referencing a DFF, flag or any attribute.