My original choice for the spectrograph’s geometry turned out to be much worse than I had even dared expect. The difference between the angle of incidence and the angle of the diffracted light was far too great.
I (obviously) didn’t know at the time, that there’s something called the anamorphic factor.¹ The definition is as follows:
r = cos θi / cos θm
If the anamorphic factor is significantly different from 1, the image of the slit on the CCD will be deformed. I don’t fully understand the implications, but I could see that a “line” supposed to take up maybe 100µm was spread out over almost 1 mm. Ok, I might be exaggerating slightly, but it was very obvious that it was never going to work.
I spent some time not thinking about it(!?), and eventually came across T.J. Nelson’s page about a High resolution compact spectrograph, and decided that this was juuust exactly what I wanted to do.
His spectrograph is of a design proposed and theoretically treated by Gil and Simon² and first realised by Schieffer³. The following diagram is from Schieffer’s publication:
My new spectrograph is still very much in the works, but so far the 90° off-axis parabolic mirrors have arrived:
Obviously some optics are missing at this point, and the grating has a plastic-box-proxy. The problems to solve are alignment of OAPs, folding mirrors, grating and CCD, and of course some practical considerations of how to fit everything inside the Modushop GX388 box.
 Gil M.A., Simon J.M., Appl. Opt. 22 (1983), p 152-158, New plane grating monochromator with off-axis parabolical mirrors.
 Schieffer S.L. et al, Appl. Opt. 46 (2007), p 3095-3101, High-resolution, flat-field, plane-grating, f/10 spectrograph with off-axis parabolic mirrors.