Low cost, open-source analytical instrumentation has the potential to increase educational outcomes for students and enable large-scale citizen science projects. Many of these instruments rely on smartphones to collect the data, mainly because they can effectively leverage a dramatic price-to-performance ratio of the optical sensors. However, several hurdles need to be overcome for these devices to be more widely adapted. In this communication we focus on visible spectrophotometers, which are common in chemistry laboratories because of the day-to-day need for quantifying concentration. To make smartphone-based spectrometers practical for wider use, we have designed a 3D-printable spectrophotometer with a dual-beam optical geometry. This geometry allows for sample and reference data to be collected on the same photograph and thus improves the signal-to-noise ratio and reproducibility of the spectra. A universal mounting system was also developed to allow for a wide variety of smartphone form factors to be coupled to the spectrophotometer. To demonstrate potential applications of this device, two assays are reported. The first is a simple illustration of the Beer--Lambert Law with common household dyes. The second is a colorimetric nitrate assay, which shows a quantitative relationship between absorption and nitrate concentration. Kinetic data are also shown for the nitrate assay, which illustrate the long time-stability of the spectral data acquired from the device.