I have a king size hemnes bed frame from ikea, but I feel it shouldn't have been scaled to that size because only a few months after getting it, everything under the top rail of the footboard started to sag a concerning amount. These brackets make it easy to install panels to fill in the open space between the legs and make it so the footboard can't sag. A bonus to closing it off, when combined with the under bed drawers, my wife's cat can't hide under the bed and make it impossible to get her out of the apartment in an emergency, which has happened once before. 

The panels I used were random shelves in the as is section of ikea that measured ~18 mm in thickness, and were cut down to 183 mm tall to allow for felt feet to be stuck to the panels to protect the floor. The length of the panels will depend entirely on the material you have, so measure the space between the legs of your bed, and subtract 4 mm per side trim you need to print, so 2 end side trims minimum, and however many center side trims you need to splice all your panels together. To attach the brackets I used ½" #8 sheet metal screws and 1" #8 wood screws. 

If you would like to modify them, it was made in onshape as my first project outside of fusion, and can be found here:

https://cad.onshape.com/documents/29f7b31808c0db5c776b5c64/w/5b20ad309dcc6c322eeb18f1/e/46166e0350260f4d87fd6e86?renderMode=0&uiState=67a1767eb8f0ab1875973e2e