I've seen so many hot glue jobs that were not totally sealed and for sure just like the picture above your going to have problems. I remember when I got the first couple sets of Gray Ghost Amphibians for testing. I threw them into the swimming pool over night and tested them the next day. They failed because they Chinese who made them where pumping them out and not really taking the time to do a quality job. Funny when I called and told Detector Pro they had failed the Chinese's told them its because they flew them over to the USA in a unpressurized area on the aircraft....lol
I started using 2 part epoxies several years ago, I even take a small paint brush and try and paint everything with a fast dry, then I will use a slow dry epoxy to fill. The epoxy will seek out all crevices, nooks and total seal for flows.
Something else some over look when it comes to sealing, is making sure the surface is clean. I tested a set of head phones for Fisher years ago. I caught a leak around the piezos on several sets they sent, come to find out I noticed the company that made the shells for the head sets use a release agent for the ear cups.. whom ever made the head phones never wiped or clean the residue off, so everything that was applied, which was a silicon sealent, could not get a grip..so they leaked.