You might also be interested in the "open source spectrophotometer"-
> The point here is if you want to do cell titers, forget OD as a fixed
> unit of measure and just calibrate it with a cell viability curve and
> go from there.
There's also the Klett colorimeter, which measures transmission of a
filtered light (not single wavelength, like OD). Klett is routinely
measured straight through a culture tube, avoiding the need to sample
your culture. The one I've used looks like it was built pre-1950, but
something similar might be an easy DIY build.