In order to improve the acoustic isolation of the box I used rubber tubing cut as depicted in Attachment 1 and attached to the edge of the plastic box (as seen in Attachment 2).

Attachment 2 also shows one of the four dog clamps I used to secure the box. Finally I placed an oscilloscope on top of the box to provide some weight.

The box has 4 holes for the input, reflected, transmitted-to-PD, and transmitted-to-camera beams.

PDH signal

In elog 2731, the PDH signal is almost 1 V pk-pk, but recently I had only been seeing a very noisy signal ~10 mV pk-pk (purple trace in Attachment 4. Transmitted light is yellow, reflected light is blue). I adjusted the lengths and removed the attenuators in the path of the EOM but it did not seem to change.

While going through the path of the cables once more, I realized that signal from the OCXO was being sent to the wrong port on the rack mount (Attachment 5). It was earlier on D3, and I later changed it to A4 which was labelled 'S EOM', but the actual location was D4 which I identified from this diagram. Now I see a reasonably large PDH signal as expected with ~800mV pk-pk (video here).