When I used to work for a power company, they used a 'tape' that was
like a strip of rubber just under maybe 1/8" thick. When you pulled it off
the roll, you had to remove a paper backing, then 'stretch' the tape to
activate it. Then when it was wrapped around a connection, it would
'cure' into a solid mass.

I think RS used to sell something similar, but have no idea what it would
be called.

You might try some industrial electrical supply houses.

Mike