I've had really good luck putting them in cardboard boxes, with simple heavy cardstock sheaths for open bay wings, then flooding the box with styrofoam peanuts.
I've shipped planes via FEDEX, on airliners and in moving trucks all the way cross country this way without a broken part yet.
It's simpler than fitted boxes, it's very lightweight which means less chance of being dropped, and cheap.
It's a pain to get them in and out on a regular basis, that's the only drawback I've found.
Making the boxes is pretty easy if you use mattress boxes to construct them. I recommend using cheap duct tape to assemble the box. Packing tape works alright, but will eventually just fall off it they're going to be in storage for a length of time.
I've put as many as four or five planes in one box. Just as long as some peanuts gets between them when they're packed, they'll do just fine.
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