For the Namespace - I would focus on the standard framework guidelines for namespace names. Put the extension methods into a namespace where they will typically be used/associated meaningfully, and avoid having an extra namespace just for this.
For the sponsor class - in this case, it's fairly unimportant. I would try to pick a class name that is meaningful, but there does not seem to be a fixed guideline.
The important thing here, though, is that the sponsor class is never really directly used/seen by the user of your extension methods. As long as the namespace has been included, the extension method is found correctly. I personally use something very similar to jrummell for my extension methods, but Microsoft does not follow this in the Framework (a good example of this is the Enumerable class).
