The hopper line mine work, although I would have to be there either way to retrieve the items when he lands, or he would just follow the hopper line back to the storage chest and take the items. Also that platform is massive (so that if he tried to move and land it the water, he couldn't) - Thats a lot of hoppers.
If you want to preserve the loot, you can add hoppers? Like for example: Eil goes to his skybase, then I do the platform thingy, then I get hoppers to auto collect and send through a redstone single of chesty things to a chest to store their items? OH maybe you can put lava instead of stone and do the same thing? Idrk just thinking of ideas