Need a non-adhesive sealant.

You are probably over think this. The gap between the tubes is small enough that whatever sealant used won't be able to be pushed in far. An Alfa knife will cut through it easily enough.

 

For sure a bit true. Gonna go google alfa knife.

 

My apologies for getting snippy. I went down for the count yesterday. Not the heart, but back problems.

 

Get well soon.

 

Yes do get well soon.
My bad it's OLFA not Alfa.
Thin retractable snap away blades.

 

Use a backer rod or some similar flexible foam filler to limit the depth of the caulk, topped off with a thin, even bead of ADHESIVE caulk, 5200 or whatever.
As long as the caulking bead is kept thin, it can be easily cut away in the future if need be.

 

I think Mr . E is going to get the W here. I ordered plasticine this week and the only question is will it stay if beat by waves? I have decided the best way is to bead around this thing, rather than insert much. The silencing is too close to the edges to get much depth into the seam.

 

Ugh. Plasticine says it is made from calcium salts. I have been researching its water resistance.

 

Toiletbowl wax is looking attractive... there's a sentence I never thought I'd write.