By combining density-functional theory and a quantum-thermodynamic model, we compute the usable hydrogen storage capacities of slit pores formed by two Li-decorated layers of the strongly anisotropic Pmmn8 borophene sheet. Our results show the important role played by the rotational degree of freedom of the hydrogen molecule in determining the confining potential within the slit pores and their hydrogen storage capacities. A remarkable finding is that for small pore widths of around 6 Å, the usable volumetric capacity of these borophene-based adsorbents at room temperature reaches the target stipulated by the U.S. Department of Energy for a quite low loading pressure of about 6 MPa when the depletion pressure is set to 0.1 MPa, which could be very useful for practical applications. Our results also show that a good usable gravimetric capacity can be reached only when the pore widths are of at least 30 Å, and operating at very large loading pressures of at least 60 MPa and temperatures lower than 100 K.