I have an S50 not the newer Stiletto but the mounting could be very similar. I haven't hooked up the S50 on the new ST yet, but I did try it in the trunk of my GL1800A. Problem was even the smallest bump would "disconnect" the S50 from the car mount and the radio would stop working.
At Americade I saw another GL1800 that had a S50 mounted on the handlebars using the car kit mounted to the standard flat round RAM mount.
I asked the rider how he kept the radio from popping off the car kit and he showed me his home made retainer. He made a stainless steel "strap", about an inch wide with a curve at the top the matched the profile of the top of the S50. The bottom of the strap matched the profile of the bottom of the car kit. With the S50 in place he would install the strap by sliding the bottom on, the strap went up behind the S50 and then with a little bending it snapped over the top of the S50 keeping it in place.
Pretty slick and simple idea that looks like it would work pretty well. Haven't tried it yet and I'm still concerned about water, I get caught in the rain alot.