I saw 2 milk crates in a storage closet at work. They have nice sized holes in the side that would let minnows in but keep larger fish out. You could stack them as in this picture.
Also, if you put a sheet of thin styrofoam in the bottom of the crate, then tipped the crate or 2 stacked crates upside down and put it in the shallows the FHM could spawn on the underside of the styrofoam. You would still want to weigh or tether the crate to keep from moving or get thin enough styrofoam that you don't float the crate.
Not so cheap to buy, but I know at our school that there were frequently broken crates (crack in the bottom or side) that caused them to be thrown aside or thrown away. Call your local school to see if they have some cracked ones in the kitchen/milk storage area or call the dairy and see if you can take the cracked ones off their hands or buy them for a low price.
