Use of a 430nm bandpass filter improves the beerometer 430nm wavelength resolution to 10 +/- 2nm. Preliminary tests with the bandpass filter showed that using the filter increased absorbance. Beer color measurements are close to the values observed with a commercial spectrophotometer.
Design files for the bandpass filter holder are in the Beerometer project on Bitbucket: https://bitbucket.org/iorodeo/beerometer
Basically, the bandpass filter is composed of three parts - the 430 nm filter and two 1/4" black acrylic pieces. These are shown in the image below - the upper holder is on the left with the filter and the lower holder on the right.
The acrylic filter holders have a large cutout for the 430nm filter and a smaller round cutout for the 700 nm LED.