It's completely realistic and normal that the temp inside that box would be 130 degrees F after a 15 minute idle with no air movement.
My guess is that on the road that temp will be more like 90 degrees F......with the wind in its sails ya know.
What was the standing air temp when you did the 15 min idle?... Also a factor.
I'm about ready to pull the trigger on the Doghouse. Given its obvious position in relation to the engine I would store tools, tire repair kit, or maybe my MSR backpacking stove in there.... in other words, metal items or items that aren't harmed by 90-100 degree heat. With the tools and other metal objects that one carries on an adventure bike this is still a great addition to the 990.