The dependence of the modal density on chamber geometry is analyzed, for general unstirred reverberation chambers at relatively low frequencies of operation. An expression for the lowest usable frequency is derived based on the chamber mode density. The use of curved wave diffractors is investigated as a method for increasing the average mode density. The specific effect of corner, edge, surface and volume diffractors on the mode density and quality factor of a nominally rectangular shielded room is analyzed as a function of diffractor size, density, and operating frequency. This paper complements earlier studies on field statistics of undermoded reverberation chambers and the performance of acoustic-type diffusers in electromagnetic reverberation chambers.