Close the flue of your chimney. If your chimney doesn't already have a hood, install one. Many models simply fit over the lip of a chimney like the lid on an airtight food storage container. The flue and the hood will keep rain from leaking onto your new shelves.
Measure the width and depth of your fireplace. Cut the 1/2-inch wood sheet to match these dimensions.
Set one of your wooden strips on end in a corner of your sheet of lumber. Glue it in place. Do the same for the remaining strips, providing legs for the other three corners of your shelf. Allow the glue to dry.
Reinforce the legs by driving two finishing nails through the wooden sheet and into the end of each leg.
Set the shelf unit, legs down, inside your fireplace.
Build additional shelf units to stack on top of the one you've installed until you've filled the vertical space in your fireplace.