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.
Actually, you can have a harp foamed and crated (this is how the pedal harp companies ship these days) and that’s really preferable to an old fashioned trunk, but I wouldn’t ship the harp unless you plan to be there a long, long time (years). There are several harp stores in Belgium and the Netherlands, both of which are convenient for Luxembourg (nothing is far in Benelux):