When dealing with methods/options POCOs that involve the Rock.Model namespace (that is things like GroupType and GroupTypeService) the following rules apply.

1. Place the file that contains the POCO in Rock\Model\[Domain]\[Entity]\Options\[PocoName].cs. Example: Rock\Model\Group\GroupType\Options\GroupFilterOptions.cs
2. Place the class in the namespace that matches the folder structure. Example: namespace Rock.Model.Group.GroupType.Options { }

A similar pattern should be used for anything outside the Rock.Model namespace, though the exact formula has not been set in stone just yet. For now, plan on using a child namespace of Options to whatever namespace your method is in to store the POCO.