Until I get the acrylic tube I have gone as far as I can. I just cut off the wires to the photodiode and soldered a new cable on. This was to avoid having to do any soldering near the plastic parts. A milivoltmeter indicates that the diode works fine now - about 250 mV in my room light. The tubing should arrive Tuesday or Wednesday.
The way I hold a PAR meter in the water is with an acrylic tube, which fits snugly into a next size larger piece of tube cemented to a strip of acrylic sheet which is the bottom of the sensor and arm to get the sensor away from the influence of the tube acting as the "probe". This is how I have our local aquarium club Quantum meter set up.