this room had two doorways, one of which went unused. so it was closed off and made into storage for books and 'weapons' (you could make the whole thing a book shelf). make sure the backing is level. (ours has paneling on the the wall behind this door, so it was secrure.
next we measured and built an 'insert' outside of the door frame using 1 x 4's.... make sure you measure on the top and bottom. our house is 115 years old and nothing is square. also give yourself some room on each side to shim it in (1/16" should suffice). we used dowels on each shelf to keep the books from falling, and a peg board on the bottom half to hang items.
once it is level and secured in, place quarter round as trim to hide any gaps. then paint it a color of your choosing.
finished product! not bad for 4" of space!