More than just Styrofoam, very tight fitting front door and the hasps actually apply compression. I have two and thought after I bought them I made a bad choice when they arrived. Both have held up well.
I use the foil steam pans from Sam's club with the lids. They slide nicely and if you have the tray slammed full the top will support the bottom of the next tray. If I remember right you can get 5 of those trays in with 2 heat pellets if you chose to use them.
That's on the average 100 - 110 LBS of product. They work equally well for cold items.