自定义控件集合 - 在设计时添加项目？

I am trying to make a reusable control similar to an Outlook-style sidebar. I have a CustomPanel. I also have a CustomCollectionControl, that inherits from flow layout panel. At design time I would like to add (x) CustomPanels to my CustomCollectionControl, through the properties window.I am trying to make a reusable control similar