If you are using the correct heat shrink tubing or terminal
connections, they have a heat activated glue. If there is any "tiny
gap around the heat-shrink tube and the cable" you have done something
wrong or used an inferior product. Ditto any heavy lugs that have
"holes" or gaps in the metal when properly swaged and sealed with heat
shrink.

I see. This means I should look for the right shrink tubing that has
heat activated glue. Thanks for the tip.

Jay Chan