I have a Caberg Modus helmet, with the Caberg Pump Lining, therefor I am unable to use the regular clap kit which clips onto the helmet. I used the Sena Glued Surface Mounting Adapter which was in the box instead, and had immediately issues with it, as the double-sided tape would not stick properly to the helmet, and it would be pealing off from the helmet, after submitting a ticket to Sena, they sent two additional Glued Surface Mounting Adapters to me, which was the same case as the original mount.
So I have decided to look at some other alternatives, and I saw my GoPro holding firmly and tightly to my helmet even when doing 90 MPH, so I searched for the same double-sided tape GoPro is using for their mounts, and I was able to find this http://solutions.3m.com.au/wps/portal/3M/en_AU/VHB/Home/Products/4991/ and finally went to Amazon and bought this http://www.amazon.com/gp/product/B00MOF18NW?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00 which fits almost perfectly on the Mounting Adapter.
Now after wiggling it, and pressing on it, and going for a ride and doing 75MPH, the Sena 20S finally holds in place and it doesn't move, Now I am worried how I am going to take it off one day :o)
I would suggest to sena to start using the 3M VHB Tape 4991 as I can confirm it holds in place!