To make a simple wren guard, cut some cardboard from a box, at one of the sides where there is a 90 degree bend. Because the cardboard will be floppy after it's cut from the box, use a small metal bracket to support it. The measurement depends on the size box you have. It should be about as wide as the box, and long enough so that it obscure the view of the entrance hole from the front when hanging on the box.