Milk crates serve a purpose beyond allowing dairy vendors to carry their product from plant to supermarket. Milk crates can become very functional building blocks to create a serviceable milk crate bench.
Don't go prowling around supermarket or dairy loading docks, however, to steal these plastic interlocking crates. There are several online sources where you can legitimately purchase them without resorting to thieving: try melmat.com and containerstore.com. In addition, you can often pick up used milk crates at flea markets and neighborhood garage sales. Milk crates are especially popular with college and university students, so at the end of each school term, be on the lookout for discards and yard sales in which the crates might be had for a very reasonable price.
For each milk crate bench you want to construct, you will need two square milk crates, each measuring approximately 13 inches by 13 inches by 11 inches. You want crates that feature grid bottoms rather than solid bottoms because you will be attaching a bench top to this grated portion when you turn the crates upside down. Try to get two crates of the same color, but it that's not possible, you can always spray paint the final bench using paint suitable for adhering to plastic.
Use a 1-inch or thicker piece of plywood or other sturdy board to create a bench top that measures 12 inches wide by about 48 inches long. This piece of wood must be study enough to carry at least 150 to 200 pounds, which is the average weight the bench must be able to support. If you don't have the equipment to cut the board to the proper size, take the dimensions with you when you visit your local lumber or hardware store and have them cut and finish the wood for you.
Lay this wooden bench top on a solid surface, placing both crates side by side and upside down atop the piece of wood. Now you are going to fasten this bench top to the milk crates using two 3/8-inch bolts and nuts at either end for a total of four fastening places.
Hold the nut and bolt in place using a washer, so that when you tighten each fastening point, the wooden bench top is securely fastened by the bolt through one of the milk crate grids. Now you can turn the bench right side up so that the wooden top is on top. You can also add legs to the bottom of the milk crates fastening with a series of nuts and bolts, or you can let the crate sit flush on the floor with no formal legs.
For a more industrial look, leave the milk crate bench unpainted. But if you want the bench to match your existing decor motif or colors, you can add cotton batting and upholstery material to cover the wooden bench top for a more finished appearance.