Sand four 36-by-36 inch hardwood pallets with a palm sander and 80-grit sandpaper. Make sure to sand down all surfaces. Finish with 200-grit sandpaper. Wipe away all sawdust with a damp cloth.
Apply a clear spray varnish to each pallet. Allow them to dry then apply a second coat. Allow that coat to dry.
Set two pallets end-to-end to form a 36-by-72 inch structure. Attach two large hinges between the two pallets with a drill. The hinges will keep the pallets from separating when the couch is complete. Repeat with the other two pallets.
Covering the Mattresses
Lay a queen-sized bedspread on a work surface, right side down. Choose a thin bedspread rather than a thick comforter for this project.
Center one twin-sized mattress over the bedspread. Fold one short end of the bedspread over the mattress and use safety pins to secure it in place. Repeat with the other short end.
Fold a long end of the bedspread over and safety-pin it in place then repeat with the other long end. The result is the mattress wrapped like a present in a bedspread. The spread should be pulled tight over the front mattress. Repeat with a second bedspread and mattress.
Putting it Together
Place one set of pallets against a corner of a wall so that it runs along one wall and abuts the other. Place the second set of pallets so that one long side runs along the second wall and one short side abuts the first set of pallets. This makes an L-shaped couch base.
Attach two large hinges with a drill between the juncture where the two sets of pallets meet. This will hold them together and keep them from separating while you're sitting on the couch.
Set one covered mattress on each of the sets of pallets. A twin mattress is generally 38-by-75 inches. The mattress will overhang the pallets lengthwise by about 2 inches, and the pallets will overhang the mattresses width-wise by about 3 inches. Pull the mattresses forward so that they are even with the front of the pallets and put throw pills along the back of the couch to fill in the space between the mattresses and the walls. Doing this will cause the second mattress to overhang the pallets lengthwise by 5 inches, instead of 2.
Stack garden bricks under the overhang on the second mattress to support it.
Have 1-by-6 inch pine boards cut to six 36-inch lengths at the lumberyard.
Set two cinder blocks on the floor at each end of the couch. The cinder blocks should each be at a corner of the mattress.
Lay a 1-by-6 board across the blocks. Top it with two more cinder blocks and a second board. Repeat with a third layer of blocks and board. The top board serves as a side table or armrest while the first and second boards are used as bookshelves.