I like B - My thoughts are suspend it under the roof top tent with 2 long pieces of C channel going from front to back. Put a stopper of some sort on the front and a removable keeper at the back. Simple slide it in thru the rear. Some sort of security is necessary.
You could also build a basic box out of 1/4" plywood. Simply bolt it to the bottom of the Thule rack. Spray it with a black bed liner product. My guess is the box would weight 15 pounds.
Looking forward to seeing what you come up with.
2012 Surly Moonlander
2012 Nimbus Oregon
2013 Jeep Wrangler Rubicon