You probably will be better off using a small diecast box, and using some aluminium cased resistors inside the box, attached to the case to heatsink them. Then you can simply use a waterproof cable gland and thermal compound between the resistors and the box, and if you want use some silicone grease on the lid to seal it.
I take it you are retrofitting LED lamps to a vehicle with lamp failure detection, so you have to handle the power of the original lamps, around 30W per side. You probably will need to use 50W metal cased resistors to handle the power.