I recently ran across this published paper. Most skunkwork types seem to buy used equipment via ebay. This article explains how to build a spectrophotometer with schematics, illustrations, and photos. The circuit is simple: a photoresistor, op amp, and some mechanics for the optics.
The article even includes a Bill of Materials (component price list); for the electronics, anyway, and it’s cheap (less than $20 for single quantities through Digikey). The article is geared towards having undergraduate students build their own laboratory equipment as an educational exercise — and if undergrads can do it, anyone can do it (the article says that a science camp of kids aged 13 to 16 found success).
Take a 100 W light bulb, a light-dependent resistor and op amp, a prism or grating in front of a slit, and a curtain – and voilà , a DIY spectrophotometer.