Not sure if it's the exact same problem, but I joined CodeProject (in 2010) to have a similar question answered (things went downhill from there). See my first ever post on CP: Groupbox in SplitContainer UserControl[^] Henry Minute (who, unfortunately, passed away since) answered with one of his own articles that may be what you want: Designing Nested Controls[^] Later, I wrote some custom controls for work that allowed you to add controls to another control (both design-time and runtime) and even add them from the properties menu after which they were automagically added to your form (like columns of a datagridview). I haven't done any serious WinForms in years though, and I switched jobs a few times and don't have that code anymore, so I can't help you more than this. Hope it helps.
Best, Sander Azure DevOps Succinctly (free eBook) Azure Serverless Succinctly (free eBook) Migrating Apps to the Cloud with Azure arrgh.js - Bringing LINQ to JavaScript