What can I use that is lightweight, but I can make to the size of about a refrigerator box?

I have a disability and can’t lift very much. I wanted to dress up and be the Tardis this Halloween to my boyfriend’s Dr Who. However, I’m realizing that using cardboard likely won’t work for me. I don’t think I’ll be able to comfortably lift it for very long. Is there a type of foam I can use to still get that large, boxy effect, but have the whole costume still be less than 10-15lbs?