Millboard is what you want. No spacers needed. Here's one manufacture's description:
"InterSource offers a premium-grade, thin, dense ceramic fiber
millboard product, which has excellent insulating properties and compressive strengths throughout its temperature use range. Our ceramic fiber millboard is produced in a unique, continuous board - manufacturing process from a blend of ceramic fibers, clay, insert fillers, and a small amount of organic/inorganic binders for increased handling strengths.
TaoFibre® ceramic fiber millboard maintains its strength and thermal stability throughout a wide temperature range up to 2300 ºF