I think the double packing in an old hardside suitcase is the safest bet. It provides extra protection and is more discreet i.e. less tempting.

I would take date stamped digital pics of the suitcase and its contents so you can prove what was in there and that it was well packed in case something should happen to it. Also have them place a fragile sticker on the suitcase.

If you can swing it, buy a Pelican case and send it in that. This will guarantee a safe transport and can be re-used for similar endeavors in the future.