Instant Furniture

Step 4 Crate Clamp

Here's one way to keep your milk crate shelves from slumping.

These chunks of aluminum extrusion press-fit over two adjacent crates.
They keep the structure rigid.

I cut these from longer extrusions I bought at a scrapyard.
At the end of the shelf you could use these to clamp the milkcrates to a vertical sideboard for extra structure. If you worry about earthquakes you could anchor that to the wall.

Actually, milk crates seem to slump a lot less than they used to.
Maybe it's time to return yours and borrow some new ones.