If you want the "nuclear option", do what Jerry said and use irradiated polyolefin meltable inner wall heat shrink tubing. Literally designed for nuclear power station penetrator assemblies. Adhesive melts inside and fills and seals. I worked on TVA and APS projects/electrical penetrator assemblies. To get power in and out of a reactor building without leaks. Developed by Raychem for this purpose. Sometimes called cross linked heat shrink tubing. You must size it carefully. Some are 2:1 and some are 4:1 shrink ratio. Too big, it won't shrink tight, too small, it will split when heat cycled in your truck. 2X wire OD or 4X wire OD...... +/- a little. The type I mention is, and has been, available for general use.
Even with heat shrink, I sometimes use liquid tape over it all if it's out of sight or in a loom.
