There is no strain on the cables as everything is pretty much tied down. I recently bought a repo mobile home to fix up and retire in. They have some sealant that they goop around pipes and wires that is almost impossible to cut thru. I could probably 'cast' some of this around that cables.
Thinking about casting I used to make molds to cast partholding fixtures with devcon mix. I could give my kid a drawing and have her 3d print a mold up. I was always using a durometer 90 rubber but thet make softer.
I was also considering the bulkhead fittings but then I would have to seal up the other slots. I just haven't given much time to a good solution.