Fibercement to reduce weight in rocket stoves

Ernie & Erica - have you come across anyone yet who has used fiberglass, vermiculite, or other lightweight amendments in their cob/cement to reduce the weight of a RS? I'd really love to add one to my trailer, but am hesitant because of the weight issue of the mass. Running the exhaust under the floor like a traditional Kang also came to mind, but I only want to rebuild this thing once and not blow the tires. Any thoughts?

Thanks so much for giving your time to the forum!