NickMow, I suggested the door seals for the fact you mentioned ... They are soft.
Relatively easy to obtain without much expense ... even less if you borrow you mates car and have a pair of scissors handy!
Seriously,the soft rubber should mould within each other, meaning, you will need two surfaces of the same to contact.
I'm not sure what you have to work with or whether this is available to do so within your model.
Secondly, I would be quite confident in saying you will need to tighten up the servo as far as possible but more so by using trim so as not overstrain.
It will be all trial and error.
Matey, just taking guesses here as I do not know what equipment you are using.
Aviation is much more my scene, and just thinking at this momement, they make RC Subs ... Maybe if you can look at one to check out what they use to seal internals of them. Youtube is a great resource.
As a thought ... No such thing as a problem, only solutions!
Best of luck mate and I will still keep thinking. Any idea is better than none.
Cheers
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One Flash.......and ur Ash!!