I generally use Raytech Magic Gel, but it is not cheap but re-enterable and can be partially removed & then topped up (and also reuse the bits you remove, if need be). Better to buy e.g. two x 500ml bottles, which cost a little under £50, and will do a number of smaller e.g. Wiska 206 or 308 junction boxes. You can buy smaller quantities but then it gets even more expensive.
If you want hard curing resin, maybe someone else can suggest something.