Here we'll be mounting the optical sensor breakout board in addition to all the electronics that's already present in our reef-pi build. For this, start with locating the dfrobot breakout board at the bottom of the housing. I generally wall mount the reef-pi housing and prefer to keep all connectors (temperature sensor, pH sensor, power strip connector, etc.) at the bottom panel of the housing.

Draw the outline of the breakout board along with locations for standoffs, and then drill holes using a Dremel or drill driver. The optical sensor breakout board uses a 4 pin JST connector, hence we'll make a rectangular hole for it. I use a file to smooth out the edges after making the initial hole with a Dremel.

Ignore the other two round holes present on the right-hand side of the housing for the time being, they are made to accommodate an extra temperature sensor and light controller connector. They will be used in next build. I was lazy, so I did the drilling in one step :-)

In addition to the rectangular hole for the optical sensor's JST connector, drill holes for fixing the breakout board with nylon screws. The breakout board has two holes for mounting:

Wire everything up. Notice I have passed the optical sensor breakout board 3 wire ribbon from beneath the Perma-Proto HAT. Make sure 5V (Red), GND (black) and GPIO 17 (green) wires are connected correctly.