Parallel femtosecond refractive index laser inscription of clinical grade poly(methyl methacrylate) (PMMA) at 775 nm, 170 fs pulselength is demonstrated with multiple low fluence beams generated with the aid of a spatial light modulator. Using optimised computer-generated holograms (CGHs), 16 diffracted near identical beams were focused simultaneously within bulk PMMA to create a series of 19 μm pitch, 5 mm×5 mm×1-4 mm thick volume phase gratings at high speed. First order diffraction efficiency rises with grating thickness in accord with diffraction theory, reaching 75\% at the first Bragg angle (4 mm thick) with fabrication time around 1 hour. By carefully stitching filamentary modifications while eliminating effects such as pulse front tilt during inscription, gratings exhibit high uniformity, which has not been achieved previously using femtosecond inscription. Highly uniform modification is exhibited throughout the material consistent with the observed excellent angular selectivity and low background scatter and quantitative comparison with first order diffraction theory is satisfactory. The diffraction efficiency and hence refractive index profile shows a temporal behaviour related to the material response after exposure. Simultaneous 3D modification at different depths is also demonstrated, highlighting the potential of creating complex 3D integrated optical circuits at high speed through the application of CGHs. \circledC 2010 Springer-Verlag.