Have you checked that both surfaces are perfectly flat and uniform with no scratches etc. ?
Both surfaces need to be parallel for Silicone selants to work (Im pretty sure this varys between selants but the ones i use do not require time to cure before torqueing up.)
You dont want to use expoxy to seal things, thats for joining things perminantly
Despite all that for plastic tops you need to be using a o-ring or gasket to be safe.