This product was mentioned in
with an average of
I found a great way to do this recently - bean bag chair beans.
Find a box approximately the right size, put the figure in it, then fill it with bean bag beans. They're not actually beans,obviously, but are tiny foam balls. They're like mini packing peanuts. They fill in the space and conform to the figure and support it in the box.
When you unpack, just pull out the figure and carefully collect the beans. They are staticky and stick.
Here are the ones i used: Comfort Research UltimaX Beans Refill, 100 Liters [link]
I bought bean bag filler to put in mine, got it on Amazon [link]
I have to add more every few months as the beans get "squished". It's very comfortable!