I got some huge triple wall cardboard boxes from my work place this week. These are realy thick and am going to use them under my compost pile to prevent the tree roots from invading the bottom of the pile. My compost piles are in a mixed wooded area and have always had problems with so many roots, the bottom of the pile is almost unusable, except to add to a new pile and it's usually the bottom that composts the fastest because I don't turn the pile very often if at all anymore.

Where did you find the slip sheets? I mean where in Costco?

Any of the areas where the products are on pallets are good places to look for the slip sheets. Sometimes you have to slip them out from under a few boxes of products. Often you will find some up on edge between the pallets or between the shelving legs and the pallets. I've also found good cardboard in the big bins the employees roll up and down the aisles to collect the empty boxes and slip sheets.

They are a great size/shape for laying out beds.

I tore open a couple leaf bags for the bottom of my pile.

:)