You might be able to borrow a trunk for your pedal harp. Get a wooden one, not one of the old fibreglass ones, so any damage will be immediately apparent. As far as I know, there are no trunks for Preludes, although you could purchase one of Lyon & Healy’s shipping cartons. Such cartons are also sold for pedal harps.
Frankly, if traveling by air, I would NEVER fly my harp anywhere on any carrier. Set up a rental at your destination if at all possible.