Use Sikoflex 221 - that's the stuff you need. Make sure it is warm before you apply it, try dunking it into a bucket of hot water for 30 mins before use, or leave it on top of a radiator, turning it every 10 minutes for half an hour before even trying to apply.
Its hard to squeeze out if its cold, in fact it wont come out. Use a quality Caulking gun too, a cheapo will just fail.
Sikoflex is the best stuff for the job. Try to find some other uses for it too, as it does not keep well after opening. I used it to seal in my 3 lens rear lights. Also did around my snorkel with it.
Sundance