Years ago I worked for a library system in Marietta, Ohio. We retired the old card catalogs in all of our branches to make way for a computer system. That left something on the order of a couple million typed 3X5 cards to dispose of. I grabbed a couple thousand and took them home. The are great for scrap, making notes, all sorts of stuff. Unfortunately I am down to my last thousand cards and will run out in a year or so. Sad sad sad....
Once while running a game of the Star Trek RPG, I needed a Borg cube for a starship combat. I just painted a wooden block black and left it at that.
An old DM of mine used to make a milk crate fort as his DM screen. It had all of his books lined up and worked great. Of course we could barely see him, but no matter, it worked for us.