This article is not directly related to DNP but describes some where nice concepts to make variable pitch NMR coils.
Kelz, Jessica I., John E. Kelly, and Rachel W. Martin. “3D-Printed Dissolvable Inserts for Efficient and Customizable Fabrication of NMR Transceiver Coils.” Journal of Magnetic Resonance 305 (August 2019): 89–92.
We describe a simpliﬁed method for improving the reproducibility of transceiver coil fabrication for nuclear magnetic resonance (NMR) through single-use templates made from 3D-printed polymer forms. The utility of dissolvable inserts for achieving performance enhanced resonators (DIAPERs) is tested herein by a comparison of RF homogeneity along the rotor axis for variable-pitch solenoids with different inter-turn spacing. Simulated B1 ﬁeld proﬁles are compared to experimental homogeneity measurements, demonstrating the potential of this approach for making NMR coils quickly and reproducibly.