Continued from last post:
Use black electricians plastic tape to hold the top lens in place, and black out the sides of the sensor, for calibration.
With the sensor shimmed up to match the height of the Quantum meter sensor, measure the PAR from a fixed light with both meters. They should match.
They do match! So, all that's left is to cement the lens in place, put acrylic cement over the cable to housing joint to waterproof it, and cement the socket for a wand to hold the sensor at the other end of the bottom strip.
The last step is to paint the exterior of the sensor housing with black nail polish, two coats, to block all light except that that comes in from the top. You now have a usable PAR meter!