and the other other solution is to use araldite.Ok I know you have done the job now but you could have just bought the correct part from a main dealer. I bought the genuine item when I last did my heater and it really is cheap (just a couple of £). Being foam-rubber it won't absorb water, is self adhesive and will last for many years before going hard.
The other 'solution' is to use silicon but you try removing the heater unit when it has been sealed in with that stuff!
Oh dear!
I don't think Araldite would work very well either.
Where did you get the genuine stuff from though.