I think I'm going into one of those brain freeze modes again.

Just put a spare fridge in the garage and it's hotter than hell out there, so it runs a lot.

I'm not going to put too much in it and it's a 21 cu ft, so I thought I'd reduce the load by putting boxes or something in it.

The question I've got, is whether it's a refrigerator or a building a/c, which is less load, a box with nothing in it, or a solid square object of the same size? In the latter case, what would be the most efficient material for the solid square object to be made of?