A 6-ohm resistor should work nicely, same idea as putting in a resistor when you install switchback turn signals. Either solder/splice it across the wires, or maybe even connect it to the base of an old 3057 bulb and plug it in so it can be swapped out quicker.
If you're going to be sending a signal to that socket still (turn signals or hazards), you may want to go for one rated for 50W, or at least 35W, because they can get hot after feeding bulb power to them for extended periods of time. Diode Dynamics has them actually, basically just a 6-ohm resistor with a big heatsink integrated.