I have created a new Sample render plug-in that adds a custom viewport properties page. The Sample project is written in C# and uses the ETO UI framework. It contains one viewport properties page and two collapsible sections.
The sample project can be downloaded from github:
https://github.com/mcneel/Rhino6Samples_CS/tree/master/SampleViewportPropertiesETOUI
More information about ETO UI framework can be found from: