This is a very nice review of cavities that are used in EPR, NMR and MRI. So far resonators have not been widely employed in DNP spectroscopy - only in some static DNP experiments. However, it is an intriguing problem that could, if solved, allow using cost-effective solid-state sources for DNP even at high temperatures.
Even if this article is not specifically about resonators for DNP it gives a very nice overview of the concepts that drive resonator design for magnetic resonance applications.
Webb, A., Cavity- and waveguide-resonators in electron paramagnetic resonance, nuclear magnetic resonance, and magnetic resonance imaging. Prog Nucl Magn Reson Spectrosc, 2014. 83C: p. 1-20.
http://www.ncbi.nlm.nih.gov/pubmed/25456314
Cavity resonators are widely used in electron paramagnetic resonance, very high field magnetic resonance microimaging and also in high field human imaging. The basic principles and designs of different forms of cavity resonators including rectangular, cylindrical, re-entrant, cavity magnetrons, toroidal cavities and dielectric resonators are reviewed. Applications in EPR and MRI are summarized, and finally the topic of traveling wave MRI using the magnet bore as a waveguide is discussed.