how do I reinforce a raised bed so its not so floppy?

I’m building a 6×12 raised bed using 4×4 cedar posts and 1″x 6′ ceder fencing. On the 12′ side, two pieces of fencing laid horizontally meet at the middle post.

Is there any way to make this wall more rigid? it bows in the center when I pick it up.

Thanks!