There’s no need to map out boxes or items that are visible, but you should list anything that gets buried on the map. You just need to get in the ballpark so there’s no need for a technical drawing- a hand drawn sketch will do fine.

Divide your unit up into four or more sections and list the items and ox numbers in each section on you map. If you happen to need a box or item that got buried in the back of the storage unit you can use your master contest list and map to find it without digging through boxes blindly.