This is more of a bump than a helpful reply. If the "moisture shield" you reference is the plastic, press fit piece sold a couple of years ago for $20, it does a good job of keeping the water out while the bike is parked. Just be careful using it because, as a press fit, it may mar a painted inner fairing. Frankly, my bike has been drenched more times than I care to remember without apparent effect on the Sony unit over the 3+ years it's been in my bike without use of the shield. And the moisture shield, at least the one I bought, doesn't stay in the inner fairing while riding. Whether the cause is vibration or vacuum, or both, it comes out while riding.
The Biketronics kit also includes a sheet of clear plastic to cover the top of the headunit inside the fairing as a water barrier.
I don't know of a discounter of the Biketronics equipment.
Hope you get some help from RG guys.