I would be very hesitant to install one and would be really surprised if they go retail. They offer a great liability to the sellers. The length added by the stand-off adds leverage to any incidental hit it takes. The plastic is not intended to be structural like that and the prying motion of incidental impact can, and will, literally pull your "plug".
Maybe I am just clumsy, but I hit my transducer on a regular basis.
Good luck if you go with it, Phill