Small enclosure. Waterproof cable entry
A flexible epoxy that you've verified to be compatible with the box material and cable jacket could work; I've used that with pretty good results in the past.  A rubber or silicone grommet with a tight fit and maybe a bit of grease could work for a round cable.  There are also heat-shrink and break-through cord grips that are somewhat less bulky than the normal compression type ones.