Best insulation for vocal booth?

I'm building a modular vocal booth that can be disassembled and reassembled using a steel frame holding inner and outer panels made of MDF. What would be the best material for the airspace cavity between the MDF panels? Ideally, it would provide maximum sound attenuation while maintaining it's shape when disassembled. This would probably rule out mineral fiber. I'm thinking rigid fiberglass panels, although I've not heard of using those in a cavity, only surface mounted. Thanks

if it itches it works