A planetarium is an invaluable tool for teaching introductory astronomy, but one that few astronomy educators have ready access to. Here we describe a do-it-yourself planetarium that can be built with modest funding. There have been other planetarium construction projects described in the literature and online, most of which use cardboard to make multiple faces, which are then fastened together to form a dome that serves as the planetarium projection surface. Our design uses a skeletal dome that supports a hemispherical cloth projection screen. A computer LCD projector, reflecting off a diverging half-dome security mirror on the interior edge of the dome base, serves as the starfield projector. The open-source sky simulation software Stellarium has the necessary features to pre-distort the sky image appropriately so that, after reflection from the diverging mirror, the dome is properly filled.