Hey all you creative crafty people... I NEED HELP!?

I need to somehow make a hollow potato the size of a microwave. I considered using paper mache but I have no idea what to mold it from. does anybody have any idea of material i could use to make a large potato looking box? if not, I need to make a box representing idaho so any additional ideas besides using a potato would be helpful as well. I just need a box to put stuff in that represents idaho in some way! Thanks!!!