To address you concern about boron, there is a cubic diamond form of boron nitride $\ce{c-BN}$, [ICSD #182731](https://www.ccdc.cam.ac.uk/structures/Search?Ccdcid=182731&DatabaseToSearch=Published) \[[1](https://doi.org/10.1088/0953-8984/23/21/215501)\], posseses $V_\mathrm{cell} = \pu{7.99 Å3}$, $Z = 2$ and molar volume

$$V_\mathrm{m} = \frac{N_\mathrm{A}V_\mathrm{cell}}{Z} = \frac{\pu{6.022e23 mol-1}\cdot\pu{7.99 Å3}}{2} \approx \pu{2.406e-6 m3 mol-1}$$

which is about $30\%$ less than diamond.
The only drawback is that this form of boron nitride is predicted to be stable above $\pu{11 Mbar}$.

[![c-BN][1]][1]

**Figure 1**. Unit cell of $\ce{c-BN}$. Color code: $\color{#FFB5B5}{\Large\bullet}~\ce{B}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$.

### References

 1. Qiu, S. L.; Marcus, P. M. Structure and Stability under Pressure of Cubic and Hexagonal Diamond Crystals of C, BN and Si from First Principles. *Journal of Physics: Condensed Matter* **2011**, 23 (21), 215501. https://doi.org/10.1088/0953-8984/23/21/215501.


  [1]: https://i.sstatic.net/gXKj6.png