Rigid foamboard is better, because it won't get moisture-soaked to the degree that fiberglass will (wet fiberglass batts lose a lot of their R value and get very heavy and saggy and awful). If you are in, like, coastal Maine and your need for insulation is only moderate, you can even try just plywood or a couple layers of bubblewrap.
All of the above can either be glued to the underside of the roof tin (use an appropriate glue if doing it on foamboard!!), or nailed/screwed/staplegunned to the underside of the rafters or purlins, or in the case of rigid materials they can be held up by cross battens. Or you can make an actual separate drop ceiling, but in most cases that's way too much work and neither necessary nor desirable.
Good luck, have fun,
Pat